Provided by: units_2.02-2_amd64 
NAME
units — unit conversion and calculation program
SYNOPSIS
'units' [options] [from-unit [to-unit]]
DESCRIPTION
The 'units' program converts quantities expressed in various systems of measurement to their equivalents
in other systems of measurement. Like many similar programs, it can handle multiplicative scale changes.
It can also handle nonlinear conversions such as Fahrenheit to Celsius. See the examples below. The
program can also perform conversions from and to sums of units, such as converting between meters and
feet plus inches.
Beyond simple unit conversions, 'units' can be used as a general-purpose scientific calculator that keeps
track of units in its calculations. You can form arbitrary complex mathematical expressions of dimen‐
sions including sums, products, quotients, powers, and even roots of dimensions. Thus you can ensure
accuracy and dimensional consistency when working with long expressions that involve many different units
that may combine in complex ways.
The units are defined in an external data file. You can use the extensive data file that comes with this
program, or you can provide your own data file to suit your needs. You can also use your own data file
to supplement the standard data file.
Basic operation is simple: you enter the units that you want to convert from and the units that you want
to convert to. You can use the program interactively with prompts, or you can use it from the command
line.
You can change the default behavior of 'units' with various options given on the command line. See Invok‐
ing Units for a description of the available options.
INTERACTING WITH UNITS
To invoke units for interactive use, type 'units' at your shell prompt. The program will print something
like this:
Currency exchange rates from 04/23/12
2516 units, 85 prefixes, 65 nonlinear units
You have:
At the 'You have:' prompt, type the quantity and units that you are converting from. For example, if you
want to convert ten meters to feet, type '10 meters'. Next, 'units' will print 'You want:'. You should
type the units you want to convert to. To convert to feet, you would type 'feet'. If the 'readline'
library was compiled in then the tab key can be used to complete unit names. See Readline Support for
more information about 'readline'. To quit the program press Ctrl-C or Ctrl-D under Unix. Under Windows
press Ctrl-Z.
The answer will be displayed in two ways. The first line of output, which is marked with a '*' to indi‐
cate multiplication, gives the result of the conversion you have asked for. The second line of output,
which is marked with a '/' to indicate division, gives the inverse of the conversion factor. If you con‐
vert 10 meters to feet, 'units' will print
* 32.808399
/ 0.03048
which tells you that 10 meters equals about 32.8 feet. The second number gives the conversion in the
opposite direction. In this case, it tells you that 1 foot is equal to about 0.03 dekameters since the
dekameter is 10 meters. It also tells you that 1/32.8 is about 0.03.
The 'units' program prints the inverse because sometimes it is a more convenient number. In the example
above, for example, the inverse value is an exact conversion: a foot is exactly 0.03048 dekameters. But
the number given the other direction is inexact.
If you convert grains to pounds, you will see the following:
You have: grains
You want: pounds
* 0.00014285714
/ 7000
From the second line of the output you can immediately see that a grain is equal to a seven thousandth
of a pound. This is not so obvious from the first line of the output. If you find the output format
confusing, try using the '--verbose' option:
You have: grain
You want: aeginamina
grain = 0.00010416667 aeginamina
grain = (1 / 9600) aeginamina
If you request a conversion between units that measure reciprocal dimensions, then 'units' will display
the conversion results with an extra note indicating that reciprocal conversion has been done:
You have: 6 ohms
You want: siemens
reciprocal conversion
* 0.16666667
/ 6
Reciprocal conversion can be suppressed by using the '--strict' option. As usual, use the '--verbose'
option to get more comprehensible output:
You have: tex
You want: typp
reciprocal conversion
1 / tex = 496.05465 typp
1 / tex = (1 / 0.0020159069) typp
You have: 20 mph
You want: sec/mile
reciprocal conversion
1 / 20 mph = 180 sec/mile
1 / 20 mph = (1 / 0.0055555556) sec/mile
If you enter incompatible unit types, the 'units' program will print a message indicating that the units
are not conformable and it will display the reduced form for each unit:
You have: ergs/hour
You want: fathoms kg^2 / day
conformability error
2.7777778e-11 kg m^2 / sec^3
2.1166667e-05 kg^2 m / sec
If you only want to find the reduced form or definition of a unit, simply press Enter at the 'You want:'
prompt. Here is an example:
You have: jansky
You want:
Definition: fluxunit = 1e-26 W/m^2 Hz = 1e-26 kg / s^2
The output from 'units' indicates that the jansky is defined to be equal to a fluxunit which in turn is
defined to be a certain combination of watts, meters, and hertz. The fully reduced (and in this case
somewhat more cryptic) form appears on the far right.
Some named units are treated as dimensionless in some situations. These units include the radian and
steradian. These units will be treated as equal to 1 in units conversions. Power is equal to torque
times angular velocity. This conversion can only be performed if the radian is dimensionless.
You have: (14 ft lbf) (12 radians/sec)
You want: watts
* 227.77742
/ 0.0043902509
Named dimensionless units are not treated as dimensionless in other contexts. They cannot be used as
exponents so for example, 'meter^radian' is not allowed.
If you want a list of options you can type '?' at the 'You want:' prompt. The program will display a
list of named units that are conformable with the unit that you entered at the 'You have:' prompt above.
Conformable unit combinations will not appear on this list.
Typing 'help' at either prompt displays a short help message. You can also type 'help' followed by a
unit name. This will invoke a pager on the units data base at the point where that unit is defined. You
can read the definition and comments that may give more details or historical information about the unit.
(You can generally quit out of the page by pressing 'q'.)
Typing 'search' text will display a list of all of the units whose names contain text as a substring
along with their definitions. This may help in the case where you aren't sure of the right unit name.
USING UNITS NON-INTERACTIVELY
The 'units' program can perform units conversions non-interactively from the command line. To do this,
type the command, type the original unit expression, and type the new units you want. If a units expres‐
sion contains non-alphanumeric characters, you may need to protect it from interpretation by the shell
using single or double quote characters.
If you type
units "2 liters" quarts
then 'units' will print
* 2.1133764
/ 0.47317647
and then exit. The output tells you that 2 liters is about 2.1 quarts, or alternatively that a quart is
about 0.47 times 2 liters.
If the conversion is successful, then 'units' will return success (zero) to the calling environment. If
you enter non-conformable units then 'units' will print a message giving the reduced form of each unit
and it will return failure (nonzero) to the calling environment.
When you invoke 'units' with only one argument, it will print out the definition of the specified unit.
It will return failure if the unit is not defined and success if the unit is defined.
UNIT DEFINITIONS
The conversion information is read from a units data file that is called 'definitions.units' and is usu‐
ally located in the '/usr/share/units' directory. If you invoke 'units' with the '-V' option, it will
print the location of this file. The default file includes definitions for all familiar units, abbrevia‐
tions and metric prefixes. It also includes many obscure or archaic units.
Many constants of nature are defined, including these:
pi ratio of circumference to diameter
c speed of light
e charge on an electron
force acceleration of gravity
mole Avogadro's number
water pressure per unit height of water
Hg pressure per unit height of mercury
au astronomical unit
k Boltzman's constant
mu0 permeability of vacuum
epsilon0 permittivity of vacuum
G Gravitational constant
mach speed of sound
The standard data file includes atomic masses for all of the elements and numerous other constants. Also
included are the densities of various ingredients used in baking so that '2 cups flour_sifted' can be
converted to 'grams'. This is not an exhaustive list. Consult the units data file to see the complete
list, or to see the definitions that are used.
The 'pound' is a unit of mass. To get force, multiply by the force conversion unit 'force' or use the
shorthand 'lbf'. (Note that 'g' is already taken as the standard abbreviation for the gram.) The unit
'ounce' is also a unit of mass. The fluid ounce is 'fluidounce' or 'floz'. British capacity units that
differ from their US counterparts, such as the British Imperial gallon, are prefixed with 'br'. Currency
is prefixed with its country name: 'belgiumfranc', 'britainpound'.
When searching for a unit, if the specified string does not appear exactly as a unit name, then the
'units' program will try to remove a trailing 's', 'es'. Next units will replace a trailing 'ies' with
'y'. If that fails, 'units' will check for a prefix. The database includes all of the standard metric
prefixes. Only one prefix is permitted per unit, so 'micromicrofarad' will fail. However, prefixes can
appear alone with no unit following them, so 'micro*microfarad' will work, as will 'micro microfarad'.
To find out which units and prefixes are available, read the standard units data file, which is exten‐
sively annotated.
English Customary Units
English customary units differ in various ways in different regions. In Britain a complex system of vol‐
ume measurements featured different gallons for different materials such as a wine gallon and ale gallon
that different by twenty percent. This complexity was swept away in 1824 by a reform that created an
entirely new gallon, the British Imperial gallon defined as the volume occupied by ten pounds of water.
Meanwhile in the USA the gallon is derived from the 1707 Winchester wine gallon, which is 231 cubic
inches. These gallons differ by about twenty percent. By default if 'units' runs in the 'en_GB' locale
you will get the British volume measures. If it runs in the 'en_US' locale you will get the US volume
measures. In other locales the default values are the US definitions. If you wish to force different
definitions then set the environment variable 'UNITS_ENGLISH' to either 'US' or 'GB' to set the desired
definitions independent of the locale.
Before 1959, the value of a yard (and other units of measure defined in terms of it) differed slightly
among English-speaking countries. In 1959, Australia, Canada, New Zealand, the United Kingdom, the
United States, and South Africa adopted the Canadian value of 1 yard = 0.9144 m (exactly), which was
approximately halfway between the values used by the UK and the US; it had the additional advantage of
making 1 inch = 2.54 cm (exactly). This new standard was termed the International Yard. Australia,
Canada, and the UK then defined all customary lengths in terms of the International Yard (Australia did
not define the furlong or rod); because many US land surveys were in terms of the pre-1959 units, the US
continued to define customary surveyors' units (furlong, chain, rod, and link) in terms of the previous
value for the foot, which was termed the US survey foot. The US defined a US survey mile as 5280 US sur‐
vey feet, and defined a statute mile as a US survey mile. The US values for these units differ from the
international values by about 2 ppm.
The 'units' program uses the international values for these units; the US values can be obtained by using
either the 'US' or the 'survey' prefix. In either case, the simple familiar relationships among the
units are maintained, e.g., 1 'furlong' = 660 'ft', and 1 'USfurlong' = 660 'USft', though the metric
equivalents differ slightly between the two cases. The 'US' prefix or the 'survey' prefix can also be
used to obtain the US survey mile and the value of the US yard prior to 1959, e.g., 'USmile' or
'surveymile' (but not 'USsurveymile'). To get the US value of the statute mile, use either
'USstatutemile' or 'USmile'.
Except for distances that extend over hundreds of miles (such as in the US State Plane Coordinate Sys‐
tem), the differences in the miles are usually insignificant:
You have: 100 surveymile - 100 mile
You want: inch
* 12.672025
/ 0.078913984
The pre-1959 UK values for these units can be obtained with the prefix 'UK'.
In the US, the acre is officially defined in terms of the US survey foot, but 'units' uses a definition
based on the international foot. If you want the official US acre use 'USacre' and similarly use
'USacrefoot' for the official US version of that unit. The difference between these units is about 4
parts per million.
UNIT EXPRESSIONS
Operators
You can enter more complicated units by combining units with operations such as powers, multiplication,
division, addition, subtraction, and parentheses for grouping. You can use the customary symbols for
these operators when 'units' is invoked with its default options. Additionally, 'units' supports some
extensions, including high priority multiplication using a space, and a high priority numerical
division operator ('|') that can simplify some expressions.
Powers of units can be specified using the '^' character as shown in the following example, or by simple
concatenation of a unit and its exponent: 'cm3' is equivalent to 'cm^3'; if the exponent is more than one
digit, the '^' is required. An exponent like '2^3^2' is evaluated right to left as usual. The '^' oper‐
ator has the second highest precedence. You can also use '**' as an exponent operator.
You have: cm^3
You want: gallons
* 0.00026417205
/ 3785.4118
You have: arabicfoot * arabictradepound * force
You want: ft lbf
* 0.7296
/ 1.370614
You multiply units using a space or an asterisk ('*'). The example above shows both forms. You can
divide units using the slash ('/') or with 'per'.
You have: furlongs per fortnight
You want: m/s
* 0.00016630986
/ 6012.8727
When a unit includes a prefix, exponent operators apply to the combination, so 'centimeter^3' gives cubic
centimeters. If you separate the prefix from the unit with any multiplication operator, such as 'centi
meter^3', then the prefix is treated as a separate unit, so the exponent does not apply. The second
example would be a hundredth of a cubic meter, not a centimeter.
Multiplication using a space has a higher precedence than division using a slash and is evaluated left
to right; in effect, the first '/' character marks the beginning of the denominator of a unit expression.
This makes it simple to enter a quotient with several terms in the denominator: 'W / m^2 Hz'. If you
multiply with '*' then you must group the terms in the denominator with parentheses: 'W / (m^2 * Hz)'.
The higher precedence of the space operator may not always be advantageous. For example, 'm/s s/day' is
equivalent to 'm / s s day' and has dimensions of length per time cubed. Similarly, '1/2 meter' refers
to a unit of reciprocal length equivalent to 0.5/meter, perhaps not what you would intend if you entered
that expression. The '*' operator is convenient for multiplying a sequence of quotients. With the '*'
operator, the example above becomes 'm/s * s/day', which is equivalent to 'm/day'. Similarly, you could
write '1/2 * meter' to get half a meter. Alternatively, parentheses can be used for grouping: you could
write '(1/2) meter' to get half a meter. See Complicated Unit Expressions for an illustration of the
various options.
The 'units' program supports another option for numerical fractions. You can indicate division of num‐
bers with the vertical bar ('|'), so if you wanted half a meter you could write '1|2 meter'. This opera‐
tor has the highest precedence, so you can write the square root of two thirds '2|3^1|2'. You cannot use
the vertical bar to indicate division of non-numerical units (e.g., 'm|s' results in an error message).
You have: 1|2 inch
You want: cm
* 1.27
/ 0.78740157
You can use parentheses for grouping:
You have: (1/2) kg / (kg/meter)
You want: league
* 0.00010356166
/ 9656.0833
Sums and Differences of Units
You may sometimes want to add values of different units that are outside the SI. You may also wish to
use 'units' as a calculator that keeps track of units. Sums of conformable units are written with the
'+' character, and differences with the '-' character.
You have: 2 hours + 23 minutes + 32 seconds
You want: seconds
* 8612
/ 0.00011611705
You have: 12 ft + 3 in
You want: cm
* 373.38
/ 0.0026782366
You have: 2 btu + 450 ft lbf
You want: btu
* 2.5782804
/ 0.38785542
The expressions that are added or subtracted must reduce to identical expressions in primitive units, or
an error message will be displayed:
You have: 12 printerspoint - 4 heredium
^
Illegal sum of non-conformable units
As usual, the precedence for '+' and '-' is lower than that of the other operators. A fractional quan‐
tity such as 2 1/2 cups can be given as '(2+1|2) cups'; the parentheses are necessary because multiplica‐
tion has higher precedence than addition. If you omit the parentheses, 'units' attempts to add '2' and
'1|2 cups', and you get an error message:
You have: 2+1|2 cups
^
Illegal sum or difference of non-conformable units
The expression could also be correctly written as '(2+1/2) cups'. If you write '2 1|2 cups' the space is
interpreted as multiplication so the result is the same as '1 cup'.
The '+' and '-' characters sometimes appears in exponents like '3.43e+8'. This leads to an ambiguity in
an expression like '3e+2 yC'. The unit 'e' is a small unit of charge, so this can be regarded as equiva‐
lent to '(3e+2) yC' or '(3 e)+(2 yC)'. This ambiguity is resolved by always interpreting '+' and '-' as
part of an exponent if possible.
Numbers as Units
For 'units', numbers are just another kind of unit. They can appear as many times as you like and in any
order in a unit expression. For example, to find the volume of a box that is 2 ft by 3 ft by 12 ft in
steres, you could do the following:
You have: 2 ft 3 ft 12 ft
You want: stere
* 2.038813
/ 0.49048148
You have: $ 5 / yard
You want: cents / inch
* 13.888889
/ 0.072
And the second example shows how the dollar sign in the units conversion can precede the five. Be care‐
ful: 'units' will interpret '$5' with no space as equivalent to 'dollar^5'.
Built-in Functions
Several built-in functions are provided: 'sin', 'cos', 'tan', 'ln', 'log', 'log2', 'exp', 'acos', 'atan'
and 'asin'. The 'sin', 'cos', and 'tan' functions require either a dimensionless argument or an argument
with dimensions of angle.
You have: sin(30 degrees)
You want:
Definition: 0.5
You have: sin(pi/2)
You want:
Definition: 1
You have: sin(3 kg)
^
Unit not dimensionless
The other functions on the list require dimensionless arguments. The inverse trigonometric functions
return arguments with dimensions of angle.
If you wish to take roots of units, you may use the 'sqrt' or 'cuberoot' functions. These functions
require that the argument have the appropriate root. You can obtain higher roots by using fractional
exponents:
You have: sqrt(acre)
You want: feet
* 208.71074
/ 0.0047913202
You have: (400 W/m^2 / stefanboltzmann)^(1/4)
You have:
Definition: 289.80882 K
You have: cuberoot(hectare)
^
Unit not a root
Complicated Unit Expressions
The 'units' program is especially helpful in ensuring accuracy and dimensional consistency when convert‐
ing lengthy unit expressions. For example, one form of the Darcy-Weisbach fluid-flow equation is
Delta P = (8 / pi)^2 (rho fLQ^2) / d^5,
where Delta P is the pressure drop, rho is the mass density, f is the (dimensionless) friction factor, L
is the length of the pipe, Q is the volumetric flow rate, and d is the pipe diameter. It might be
desired to have the equation in the form
Delta P = A1 rho fLQ^2 / d^5
that accepted the user's normal units; for typical units used in the US, the required conversion could be
something like
You have: (8/pi^2)(lbm/ft^3)ft(ft^3/s)^2(1/in^5)
You want: psi
* 43.533969
/ 0.022970568
The parentheses allow individual terms in the expression to be entered naturally, as they might be read
from the formula. Alternatively, the multiplication could be done with the '*' rather than a space; then
parentheses are needed only around 'ft^3/s' because of its exponent:
You have: 8/pi^2 * lbm/ft^3 * ft * (ft^3/s)^2 /in^5
You want: psi
* 43.533969
/ 0.022970568
Without parentheses, and using spaces for multiplication, the previous conversion would need to be
entered as
You have: 8 lb ft ft^3 ft^3 / pi^2 ft^3 s^2 in^5
You want: psi
* 43.533969
/ 0.022970568
Backwards Compatibility:
'*' and '-' The original 'units' assigned multiplication a higher precedence than division using the
slash. This differs from the usual precedence rules, which give multiplication and division equal prece‐
dence, and can be confusing for people who think of units as a calculator.
The star operator ('*') included in this 'units' program has, by default, the same precedence as divi‐
sion, and hence follows the usual precedence rules. For backwards compatibility you can invoke 'units'
with the '--oldstar' option. Then '*' has a higher precedence than division, and the same precedence as
multiplication using the space.
Historically, the hyphen ('-') has been used in technical publications to indicate products of units, and
the original 'units' program treated it as a multiplication operator. Because 'units' provides several
other ways to obtain unit products, and because '-' is a subtraction operator in general algebraic
expressions, 'units' treats the binary '-' as a subtraction operator by default. For backwards compati‐
bility use the '--product' option, which causes 'units' to treat the binary '-' operator as a product
operator. When '-' is a multiplication operator it has the same precedence as multiplication with a
space, giving it a higher precedence than division.
When '-' is used as a unary operator it negates its operand. Regardless of the 'units' options, if '-'
appears after '(' or after '+' then it will act as a negation operator. So you can always compute 20
degrees minus 12 minutes by entering '20 degrees + -12 arcmin'. You must use this construction when you
define new units because you cannot know what options will be in force when your definition is processed.
NONLINEAR UNIT CONVERSIONS
Nonlinear units are represented using functional notation. They make possible nonlinear unit conversions
such as temperature.
Temperature Conversions
Conversions between temperatures are different from linear conversions between temperature increments—see
the example below. The absolute temperature conversions are handled by units starting with 'temp', and
you must use functional notation. The temperature-increment conversions are done using units starting
with 'deg' and they do not require functional notation.
You have: tempF(45)
You want: tempC
7.2222222
You have: 45 degF
You want: degC
* 25
/ 0.04
Think of 'tempF(x)' not as a function but as a notation that indicates that x should have units of
'tempF' attached to it. See Defining Nonlinear Units. The first conversion shows that if it's 45
degrees Fahrenheit outside, it's 7.2 degrees Celsius. The second conversion indicates that a change of
45 degrees Fahrenheit corresponds to a change of 25 degrees Celsius. The conversion from 'tempF(x)' is
to absolute temperature, so that
You have: tempF(45)
You want: degR
* 504.67
/ 0.0019814929
gives the same result as
You have: tempF(45)
You want: tempR
* 504.67
/ 0.0019814929
But if you convert 'tempF(x)' to 'degC', the output is probably not what you expect:
You have: tempF(45)
You want: degC
* 280.37222
/ 0.0035666871
The result is the temperature in K, because 'degC' is defined as 'K', the Kelvin. For consistent results,
use the 'tempX' units when converting to a temperature rather than converting a temperature increment.
Other Nonlinear Units
Some other examples of nonlinear units are numerous different ring sizes and wire gauges, the grit sizes
used for abrasives, the decibel scale, shoe size, scales for the density of sugar (e.g. baume). The
standard data file also supplies units for computing the area of a circle and the volume of a sphere.
See the standard units data file for more details. Wire gauges with multiple zeroes are signified using
negative numbers where two zeroes is '-1'. Alternatively, you can use the synonyms 'g00', 'g000', and so
on that are defined in the standard units data file.
You have: wiregauge(11)
You want: inches
* 0.090742002
/ 11.020255
You have: brwiregauge(g00)
You want: inches
* 0.348
/ 2.8735632
You have: 1 mm
You want: wiregauge
18.201919
You have: grit_P(600)
You want: grit_ansicoated
342.76923
The last example shows the conversion from P graded sand paper, which is the European standard and may be
marked ``P600'' on the back, to the USA standard.
You can compute the area of a circle using the nonlinear unit, 'circlearea'. You can also do this using
the circularinch or circleinch. The next example shows two ways to compute the area of a circle with a
five inch radius and one way to compute the volume of a sphere with a radius of one meter.
You have: circlearea(5 in)
You want: in2
* 78.539816
/ 0.012732395
You have: 10^2 circleinch
You want: in2
* 78.539816
/ 0.012732395
You have: spherevol(meter)
You want: ft3
* 147.92573
/ 0.0067601492
UNIT LISTS: CONVERSION TO SUMS OF UNITS
Outside of the SI, it is sometimes desirable to convert a single unit to a sum of units—for example, feet
to feet plus inches. The conversion from sums of units was described in Sums and Differences of Units,
and is a simple matter of adding the units with the '+' sign:
You have: 12 ft + 3 in + 3|8 in
You want: ft
* 12.28125
/ 0.081424936
Although you can similarly write a sum of units to convert to, the result will not be the conversion to
the units in the sum, but rather the conversion to the particular sum that you have entered:
You have: 12.28125 ft
You want: ft + in + 1|8 in
* 11.228571
/ 0.089058524
The unit expression given at the 'You want:' prompt is equivalent to asking for conversion to multiples
of '1 ft + 1 in + 1|8 in', which is 1.09375 ft, so the conversion in the previous example is equivalent
to
You have: 12.28125 ft
You want: 1.09375 ft
* 11.228571
/ 0.089058524
In converting to a sum of units like miles, feet and inches, you typically want the largest integral
value for the first unit, followed by the largest integral value for the next, and the remainder
converted to the last unit. You can do this conversion easily with 'units' using a special syntax for
lists of units. You must list the desired units in order from largest to smallest, separated by the
semicolon (';') character:
You have: 12.28125 ft
You want: ft;in;1|8 in
12 ft + 3 in + 3|8 in
The conversion always gives integer coefficients on the units in the list, except possibly the last unit
when the conversion is not exact:
You have: 12.28126 ft
You want: ft;in;1|8 in
12 ft + 3 in + 3.00096 * 1|8 in
The order in which you list the units is important:
You have: 3 kg
You want: oz;lb
105 oz + 0.051367866 lb
You have: 3 kg
You want: lb;oz
6 lb + 9.8218858 oz
Listing ounces before pounds produces a technically correct result, but not a very useful one. You must
list the units in descending order of size in order to get the most useful result.
Ending a unit list with the separator ';' has the same effect as repeating the last unit on the list, so
'ft;in;1|8 in;' is equivalent to 'ft;in;1|8 in;1|8 in'. With the example above, this gives
You have: 12.28126 ft
You want: ft;in;1|8 in;
12 ft + 3 in + 3|8 in + 0.00096 * 1|8 in
in effect separating the integer and fractional parts of the coefficient for the last unit. If you
instead prefer to round the last coefficient to an integer you can do this with the '--round' ('-r')
option. With the previous example, the result is
You have: 12.28126 ft
You want: ft;in;1|8 in
12 ft + 3 in + 3|8 in (rounded down to nearest 1|8 in)
When you use the '-r' option, repeating the last unit on the list has no effect (e.g., 'ft;in;1|8 in;1|8
in' is equivalent to 'ft;in;1|8 in'), and hence neither does ending a list with a ';'. With a single
unit and the '-r' option, a terminal ';' does have an effect: it causes 'units' to treat the single unit
as a list and produce a rounded value for the single unit. Without the extra ';', the '-r' option has no
effect on single unit conversions. This example shows the output using the '-r' option:
You have: 12.28126 ft
You want: in
* 147.37512
/ 0.0067854058
You have: 12.28126 ft
You want: in;
147 in (rounded down to nearest in)
Each unit that appears in the list must be conformable with the first unit on the list, and of course the
listed units must also be conformable with the You have unit that you enter.
You have: meter
You want: ft;kg
^
conformability error
ft = 0.3048 m
kg = 1 kg
You have: meter
You want: lb;oz
conformability error
1 m
0.45359237 kg
In the first case, 'units' reports the disagreement between units appearing on the list. In the second
case, 'units' reports disagreement between the unit you entered and the desired conversion. This con‐
formability error is based on the first unit on the unit list.
Other common candidates for conversion to sums of units are angles and time:
You have: 23.437754 deg
You want; deg;arcmin;arcsec
23 deg + 26 arcmin + 15.9144 arcsec
You have: 7.2319 hr
You want: hr;min;sec
7 hr + 13 min + 54.84 sec
In North America, recipes for cooking typically measure ingredients by volume, and use units that are not
always convenient multiples of each other. Suppose that you have a recipe for 6 and you wish to make a
portion for 1. If the recipe calls for 2 1/2 cups of an ingredient, you might wish to know the measure‐
ments in terms of measuring devices you have available, you could use 'units' and enter
You have: (2+1|2) cup / 6
You want: cup;1|2 cup;1|3 cup;1|4 cup;tbsp;tsp;1|2 tsp;1|4 tsp
1|3 cup + 1 tbsp + 1 tsp
By default, if a unit in a list begins with fraction of the form 1|x and its multiplier is an integer,
the fraction is given as the product of the multiplier and the numerator; for example,
You have: 12.28125 ft
You want: ft;in;1|8 in;
12 ft + 3 in + 3|8 in
In many cases, such as the example above, this is what is wanted, but sometimes it is not. For example,
a cooking recipe for 6 might call for 5 1/4 cup of an ingredient, but you want a portion for 2, and your
1-cup measure is not available; you might try
You have: (5+1|4) cup / 3
You want: 1|2 cup;1|3 cup;1|4 cup
3|2 cup + 1|4 cup
This result might be fine for a baker who has a 1 1/2-cup measure (and recognizes the equivalence), but
it may not be as useful to someone with more limited set of measures, who does want to do additional cal‐
culations, and only wants to know ``How many 1/2-cup measures to I need to add?'' After all, that's what
was actually asked. With the '--show-factor' option, the factor will not be combined with a unity numer‐
ator, so that you get
You have: (5+1|4) cup / 3
You want: 1|2 cup;1|3 cup;1|4 cup
3 * 1|2 cup + 1|4 cup
A user-specified fractional unit with a numerator other than 1 is never overridden, however—if a unit
list specifies '3|4 cup;1|2 cup', a result equivalent to 1 1/2 cups will always be shown as '2 * 3|4 cup'
whether or not the '--show-factor' option is given.
Some applications for unit lists may be less obvious. Suppose that you have a postal scale and wish to
ensure that it's accurate at 1 oz, but have only metric calibration weights. You might try
You have: 1 oz
You want: 100 g;50 g; 20 g;10 g;5 g;2 g;1 g;
20 g + 5 g + 2 g + 1 g + 0.34952312 * 1 g
You might then place one each of the 20 g, 5 g, 2 g, and 1 g weights on the scale and hope that it indi‐
cates close to
You have: 20 g + 5 g + 2 g + 1 g
You want: oz;
0.98767093 oz
Appending ';' to 'oz' forces a one-line display that includes the unit; here the integer part of the
result is zero, so it is not displayed.
A unit list such as
cup;1|2 cup;1|3 cup;1|4 cup;tbsp;tsp;1|2 tsp;1|4 tsp
can be tedious to enter. The 'units' program provides shorthand names for some common combinations:
hms hours, minutes, seconds
dms angle: degrees, minutes, seconds
time years, days, hours, minutes and seconds
usvol US cooking volume: cups and smaller
Using these shorthands, or unit list aliases, you can do the following conversions:
You have: anomalisticyear
You want: time
1 year + 25 min + 3.4653216 sec
You have: 1|6 cup
You want: usvol
2 tbsp + 2 tsp
You cannot combine a unit list alias with other units: it must appear alone at the 'You want:' prompt.
You can display the definition of a unit list alias by entering it at the 'You have:' prompt:
You have: dms
Definition: unit list, deg;arcmin;arcsec
When you specify compact output with '--compact', '--terse' or '-t' and perform conversion to a unit
list, 'units' lists the conversion factors for each unit in the list, separated by semicolons.
You have: year
You want: day;min;sec
365;348;45.974678
Unlike the case of regular output, zeros are included in this output list:
You have: liter
You want: cup;1|2 cup;1|4 cup;tbsp
4;0;0;3.6280454
INVOKING UNITS
You invoke 'units' like this:
units [options] [from-unit [to-unit]]
If the from-unit and to-unit are omitted, the program will use interactive prompts to determine which
conversions to perform. See Interactive Use. If both from-unit and to-unit are given, 'units' will
print the result of that single conversion and then exit. If only from-unit appears on the command line,
'units' will display the definition of that unit and exit. Units specified on the command line may need
to be quoted to protect them from shell interpretation and to group them into two arguments. See Command
Line Use.
The default behavior of 'units' can be changed by various options given on the command line. In most
cases, the options may be given in either short form (a single '-' followed by a single character) or
long form ('--' followed by a word or hyphen-separated words). Short-form options are cryptic but
require less typing; long-form options require more typing but are more explanatory and may be more
mnemonic. With long-form options you need only enter sufficient characters to uniquely identify the
option to the program. For example, '--out %f' works, but '--o %f' fails because 'units' has other long
options beginning with 'o'. However, '--q' works because '--quiet' is the only long option beginning
with 'q'.
Some options require arguments to specify a value (e.g., '-d 12' or '--digits 12'). Short-form options
that do not take arguments may be concatenated (e.g., '-erS' is equivalent to '-e -r -S'); the last
option in such a list may be one that takes an argument (e.g., '-ed 12'). With short-form options, the
space between an option and its argument is optional (e.g., '-d12' is equivalent to '-d 12'). Long-form
options may not be concatenated, and the space between a long-form option and its argument is required.
Short-form and long-form options may be intermixed on the command line. Options may be given in any
order, but when incompatible options (e.g., '--output-format' and '--exponential') are given in combina‐
tion, behavior is controlled by the last option given. For example, '-o%.12f -e' gives exponential for‐
mat with the default eight significant digits).
The following options are available:
-c, --check
Check that all units and prefixes defined in the units data file reduce to primitive units. Print
a list of all units that cannot be reduced. Also display some other diagnostics about suspicious
definitions in the units data file. Only definitions active in the current locale are checked.
You should always run 'units' with this option after modifying a units data file.
--check-verbose, --verbose-check
Like the '--check' option, this option prints a list of units that cannot be reduced. But to help
find unit definitions that cause endless loops, it lists the units as they are checked. If
'units' hangs, then the last unit to be printed has a bad definition. Only definitions active in
the current locale are checked.
-d ndigits, --digits ndigits
Set the number of significant digits in the output to the value specified (which must be greater
than zero). For example, '-d 12' sets the number of significant digits to 12. With exponential
output 'units' displays one digit to the left of the decimal point and eleven digits to the right
of the decimal point. On most systems, the maximum number of internally meaningful digits is 15;
if you specify a greater number than your system's maximum, 'units' will print a warning and set
the number to the largest meaningful value. To directly set the maximum value, give an argument
of 'max' (e.g., '-d max'). Be aware, of course, that ``significant'' here refers only to the dis‐
play of numbers; if results depend on physical constants not known to this precision, the physi‐
cally meaningful precision may be less than that shown. The '--digits' option conflicts with the
'--output-format' option.
-e, --exponential
Set the numeric output format to exponential (i.e., scientific notation), like that used in the
Unix 'units' program. The default precision is eight significant digits (seven digits to the
right of the decimal point); this can be changed with the '--digits' option. The '--exponential'
option conflicts with the '--output-format' option.
-o format, --output-format format
This option affords complete control over the numeric output format using the specified format.
The format is a single floating point numeric format for the 'printf()' function in the C program‐
ming language. All compilers support the format types 'g' and 'G' to specify significant digits,
'e' and 'E' for scientific notation, and 'f' for fixed-point decimal. The ISO C99 standard intro‐
duced the 'F' type for fixed-point decimal and the 'a' and 'A' types for hexadecimal floating
point; these types are allowed with compilers that support them. The default format is '%.8g';
for greater precision, you could specify '-o %.15g'. See Numeric Output Format and the documenta‐
tion for 'printf()' for more detailed descriptions of the format specification. The '--output-
format' option affords the greatest control of the output appearance, but requires at least rudi‐
mentary knowledge of the 'printf()' format syntax. If you don't want to bother with the
'printf()' syntax, you can specify greater precision more simply with the '--digits' option or
select exponential format with '--exponential'. The '--output-format' option is incompatible with
the '--exponential' and '--digits' options.
-f filename, --file filename
Instruct 'units' to load the units file 'filename'. You can specify up to 25 units files on the
command line. When you use this option, 'units' will load only the files you list on the command
line; it will not load the standard file or your personal units file unless you explicitly list
them. If filename is the empty string ('-f ""'), the default units file (or that specified by
'UNITSFILE') will be loaded in addition to any others specified with '-f'.
-h, --help
Print out a summary of the options for 'units'.
-m, --minus
Causes '-' to be interpreted as a subtraction operator. This is the default behavior.
-p, --product
Causes '-' to be interpreted as a multiplication operator when it has two operands. It will act
as a negation operator when it has only one operand: '(-3)'. By default '-' is treated as a sub‐
traction operator.
--oldstar
Causes '*' to have the old-style precedence, higher than the precedence of division so that
'1/2*3' will equal '1/6'.
--newstar
Forces '*' to have the new (default) precedence that follows the usual rules of algebra: the
precedence of '*' is the same as the precedence of '/', so that '1/2*3' will equal '3/2'.
--compact
Give compact output featuring only the conversion factor. This turns off the '--verbose' option.
-q, --quiet, --silent
Suppress prompting of the user for units and the display of statistics about the number of units
loaded.
-n, --nolists
Disable conversion to unit lists.
-r, --round
When converting to a combination of units given by a unit list, round the value of the last unit
in the list to the nearest integer.
-S, --show-factor
When converting to a combination of units specified in a list, always show a non-unity factor
before a unit that begins with a fraction with a unity denominator. By default, if the unit in a
list begins with fraction of the form 1|x and its multiplier is an integer other than 1, the frac‐
tion is given as the product of the multiplier and the numerator (e.g., '3|8 in' rather than '3 *
1|8 in'). In some cases, this is not what is wanted; for example, the results for a cooking
recipe might show '3 * 1|2 cup' as '3|2 cup'. With the '--show-factor' option, a result equiva‐
lent to 1.5 cups will display as '3 * 1|2 cup' rather than '3|2 cup'. A user-specified fractional
unit with a numerator other than 1 is never overridden, however—if a unit list specifies '3|4
cup;1|2 cup', a result equivalent to 1 1/2 cups will always be shown as '2 * 3|4 cup' whether or
not the '--show-factor' option is given.
-s, --strict
Suppress conversion of units to their reciprocal units. For example, 'units' will normally con‐
vert hertz to seconds because these units are reciprocals of each other. The strict option
requires that units be strictly conformable to perform a conversion, and will give an error if you
attempt to convert hertz to seconds.
-1, --one-line
Give only one line of output (the forward conversion). Do not print the reverse conversion. If a
reciprocal conversion is performed then 'units' will still print the ``reciprocal conversion''
line.
-t, --terse
Give terse output when converting units. This option can be used when calling 'units' from
another program so that the output is easy to parse. This option has the combined effect of these
options: '--strict' '--quiet' '--one-line' '--compact'.
-v, --verbose
Give slightly more verbose output when converting units. When combined with the '-c' option this
gives the same effect as '--check-verbose'.
-V, --version
Print program version number, tell whether the 'readline' library has been included, and give the
location of the default units data file.
-l locale, --locale locale
Force a specified locale such as 'en_GB' to get British definitions by default. This overrides
the locale determined from system settings or environment variables. See Locale for a description
of locale format.
ADDING YOUR OWN DEFINITIONS
Units Data Files
The units and prefixes that 'units' can convert are defined in the units data file, typically
'/usr/share/units/definitions.units'. Although you can extend or modify this data file if you have
appropriate user privileges, it's usually better to put extensions in separate files so that the defini‐
tions will be preserved when you update 'units'.
You can include additional data files in the units database using the '!include' command in the standard
units data file. For example
!include /usr/local/share/units/local.units
might be appropriate for a site-wide supplemental data file. The location of the '!include' statement in
the standard units data file is important; later definitions replace earlier ones, so any definitions in
an included file will override definitions before the '!include' statement in the standard units data
file. With normal invocation, no warning is given about redefinitions; to ensure that you don't have an
unintended redefinition, run 'units -c' after making changes to any units data file.
If you want to add your own units in addition to or in place of standard or site-wide supplemental units
data files, you can include them in the '.units' file in your home directory. If this file exists it is
read after the standard units data file, so that any definitions in this file will replace definitions of
the same units in the standard data file or in files included from the standard data file. This file
will not be read if any units files are specified on the command line. (Under Windows the personal units
file is named 'unitdef.units'.)
The 'units' program first tries to determine your home directory from the 'HOME' environment variable.
On systems running Microsoft Windows, if 'HOME' does not exist, 'units' attempts to find your home direc‐
tory from 'HOMEDRIVE' and 'HOMEPATH'. Running 'units -V' will display the location and name of your per‐
sonal units file.
You can specify an arbitrary file as your personal units data file with the 'MYUNITSFILE' environment
variable; if this variable exists, its value is used without searching your home directory.
Defining New Units and Prefixes
A unit is specified on a single line by giving its name and an equivalence. Comments start with a '#'
character, which can appear anywhere in a line. The backslash character ('\') acts as a continuation
character if it appears as the last character on a line, making it possible to spread definitions out
over several lines if desired. A file can be included by giving the command '!include' followed by the
file's name. The '!' must be the first character on the line. The file will be sought in the same
directory as the parent file unless you give a full path. The name of the file to be included cannot
contain the comment character '#'.
Unit names must not contain any of the operator characters '+', '-', '*', '/', '|', '^', ';', '~', the
comment character '#', or parentheses. They cannot begin or end with an underscore ('_'), a comma (',')
or a decimal point ('.'). The figure dash (U+2012), typographical minus (`-'; U+2212), and en dash (`-';
U+2013) are converted to the operator '-', so none of these characters can appear in unit names. Names
cannot begin with a digit, and if a name ends in a digit other than zero, the digit must be preceded by a
string beginning with an underscore, and afterwards consisting only of digits, decimal points, or commas.
For example, 'foo_2', 'foo_2,1', or 'foo_3.14' would be valid names but 'foo2' or 'foo_a2' would be
invalid. You could define nitrous oxide as
N2O nitrogen 2 + oxygen
but would need to define nitrogen dioxide as
NO_2 nitrogen + oxygen 2
Be careful to define new units in terms of old ones so that a reduction leads to the primitive units,
which are marked with '!' characters. Dimensionless units are indicated by using the string
'!dimensionless' for the unit definition.
When adding new units, be sure to use the '-c' option to check that the new units reduce properly. If
you create a loop in the units definitions, then 'units' will hang when invoked with the '-c' option.
You will need to use the '--check-verbose' option, which prints out each unit as it is checked. The pro‐
gram will still hang, but the last unit printed will be the unit that caused the infinite loop.
If you define any units that contain '+' characters, carefully check them because the '-c' option will
not catch non-conformable sums. Be careful with the '-' operator as well. When used as a binary opera‐
tor, the '-' character can perform addition or multiplication depending on the options used to invoke
'units'. To ensure consistent behavior use '-' only as a unary negation operator when writing units def‐
initions. To multiply two units leave a space or use the '*' operator with care, recalling that it has
two possible precedence values and may require parentheses to ensure consistent behavior. To compute the
difference of 'foo' and 'bar' write 'foo+(-bar)' or even 'foo+-bar'.
Here is an example of a short data file that defines some basic units:
m ! # The meter is a primitive unit
sec ! # The second is a primitive unit
rad !dimensionless # A dimensionless primitive unit
micro- 1e-6 # Define a prefix
minute 60 sec # A minute is 60 seconds
hour 60 min # An hour is 60 minutes
inch 0.0254 m # Inch defined in terms of meters
ft 12 inches # The foot defined in terms of inches
mile 5280 ft # And the mile
A unit that ends with a '-' character is a prefix. If a prefix definition contains any '/' characters,
be sure they are protected by parentheses. If you define 'half- 1/2' then 'halfmeter' would be equiva‐
lent to '1 / (2 meter)'.
Defining Nonlinear Units
Some unit conversions of interest are nonlinear; for example, temperature conversions between the Fahren‐
heit and Celsius scales cannot be done by simply multiplying by conversion factors.
When you give a linear unit definition such as 'inch 2.54 cm' you are providing information that 'units'
uses to convert values in inches into primitive units of meters. For nonlinear units, you give a func‐
tional definition that provides the same information.
Nonlinear units are represented using a functional notation. It is best to regard this notation not as a
function call but as a way of adding units to a number, much the same way that writing a linear unit name
after a number adds units to that number. Internally, nonlinear units are defined by a pair of functions
that convert to and from linear units in the data file, so that an eventual conversion to primitive units
is possible.
Here is an example nonlinear unit definition:
tempF(x) units=[1;K] (x+(-32)) degF + stdtemp ; \
(tempF+(-stdtemp))/degF + 32
A nonlinear unit definition comprises a unit name, a dummy parameter name, two functions, and two corre‐
sponding units. The functions tell 'units' how to convert to and from the new unit. In order to produce
valid results, the arguments of these functions need to have the correct dimensions. To facilitate error
checking, you may optionally indicate units for these arguments.
The definition begins with the unit name followed immediately (with no spaces) by a '(' character. In
parentheses is the name of the parameter. Next is an optional specification of the units required by the
functions in this definition. In the example above, the 'tempF' function requires an input argument con‐
formable with '1'. For normal nonlinear units definitions the forward function will always take a dimen‐
sionless argument. The inverse function requires an input argument conformable with 'K'. In general the
inverse function will need units that match the quantity measured by your nonlinear unit. The purpose of
the expression in brackets to enable 'units' to perform error checking on function arguments, and also to
assign units to range and domain specifications, which are described later.
Next the function definitions appear. In the example above, the 'tempF' function is defined by
tempF(x) = (x+(-32)) degF + stdtemp
This gives a rule for converting 'x' in the units 'tempF' to linear units of absolute temperature, which
makes it possible to convert from tempF to other units.
In order to make conversions to Fahrenheit possible, you must give a rule for the inverse conversions.
The inverse will be 'x(tempF)' and its definition appears after a ';' character. In our example, the
inverse is
x(tempF) = (tempF+(-stdtemp))/degF + 32
This inverse definition takes an absolute temperature as its argument and converts it to the Fahrenheit
temperature. The inverse can be omitted by leaving out the ';' character, but then conversions to the
unit will be impossible. If the inverse is omitted then the '--check' option will display a warning. It
is up to you to calculate and enter the correct inverse function to obtain proper conversions. The
'--check' option tests the inverse at one point and prints an error if it is not valid there, but this is
not a guarantee that your inverse is correct.
If you wish to make synonyms for nonlinear units, you still need to define both the forward and inverse
functions. Inverse functions can be obtained using the '~' operator. So to create a synonym for 'tempF'
you could write
fahrenheit(x) units=[1;K] tempF(x); ~tempF(fahrenheit)
You may define a function whose range and domain do not cover all of the real numbers. In this case
'units' can handle errors better if you specify an appropriate range and domain. You specify the range
and domain as shown below.
baume(d) units=[1;g/cm^3] domain=[0,130.5] range=[1,10] \
(145/(145-d)) g/cm^3 ; (baume+-g/cm^3) 145 / baume
In this example the domain is specified after the 'domain=' with the endpoints given in brackets. One of
the end points can be omitted to get an interval that goes to infinity. So the range could be specified
as nonnegative by writing 'range=[0,]'. Both the range and domain are optional and can appear indepen‐
dently and in any order along with the 'units' specification. The values in the range and domain are
attached to the units given in the 'units' specification. If you don't specify the units then the para‐
meter inputs are reduced to primitive units for the numeric comparison to the values you give in the
range or domain. In this case you should only use 'range' or 'domain' if the endpoints are zero and
infinity.
Specifying the range and domain allows 'units' to perform better error checking and give more helpful
error messages when you invoke nonlinear units conversions outside of their bounds. It also enables the
'-c' option to find a point in the domain to use for its point check of your inverse definition.
You may occasionally wish to define a function that operates on units. This can be done using a nonlin‐
ear unit definition. For example, the definition below provides conversion between radius and the area
of a circle. This definition requires a length as input and produces an area as output, as indicated by
the 'units=' specification. Specifying the range as the nonnegative numbers can prevent cryptic error
messages.
circlearea(r) units=[m;m^2] range=[0,] pi r^2 ; sqrt(circlearea/pi)
Sometimes you may be interested in a piecewise linear unit such as many wire gauges. Piecewise linear
units can be defined by specifying conversions to linear units on a list of points. Conversion at other
points will be done by linear interpolation. A partial definition of zinc gauge is
zincgauge[in] 1 0.002, 10 0.02, 15 0.04, 19 0.06, 23 0.1
In this example, 'zincgauge' is the name of the piecewise linear unit. The definition of such a unit is
indicated by the embedded '[' character. After the bracket, you should indicate the units to be attached
to the numbers in the table. No spaces can appear before the ']' character, so a definition like 'foo[kg
meters]' is illegal; instead write 'foo[kg*meters]'. The definition of the unit consists of a list of
pairs optionally separated by commas. This list defines a function for converting from the piecewise
linear unit to linear units. The first item in each pair is the function argument; the second item is
the value of the function at that argument (in the units specified in brackets). In this example, we
define 'zincgauge' at five points. For example, we set 'zincgauge(1)' equal to '0.002 in'. Definitions
like this may be more readable if written using continuation characters as
zincgauge[in] \
1 0.002 \
10 0.02 \
15 0.04 \
19 0.06 \
23 0.1
With the preceding definition, the following conversion can be performed:
You have: zincgauge(10)
You want: in
* 0.02
/ 50
You have: .01 inch
You want: zincgauge
5
If you define a piecewise linear unit that is not strictly monotonic, then the inverse will not be well
defined. If the inverse is requested for such a unit, 'units' will return the smallest inverse. The
'--check' option will print a warning if a non-monotonic piecewise linear unit is encountered.
Defining Unit List Aliases
Unit list aliases are treated differently from unit definitions, because they are a data entry shorthand
rather than a true definition for a new unit. A unit list alias definition begins with '!unitlist' and
includes the alias and the definition; for example, the aliases included in the standard units data file
are
!unitlist hms hr;min;sec
!unitlist time year;day;hr;min;sec
!unitlist dms deg;arcmin;arcsec
!unitlist ftin ft;in;1|8 in
!unitlist usvol cup;3|4 cup;2|3 cup;1|2 cup;1|3 cup;1|4 cup;\
tbsp;tsp;1|2 tsp;1|4 tsp;1|8 tsp
Unit list aliases are only for unit lists, so the definition must include a ';'. Unit list aliases can
never be combined with units or other unit list aliases, so the definition of 'time' shown above could
not have been shortened to 'year;day;hms'. As usual, be sure to run 'units --check' to ensure that the
units listed in unit list aliases are conformable.
NUMERIC OUTPUT FORMAT
By default, 'units' shows results to eight significant digits. You can change this with the
'--exponential', '--digits', and '--output-format' options. The first sets an exponential format (i.e.,
scientific notation) like that used in the original Unix 'units' program, the second allows you to spec‐
ify a different number of significant digits, and the last allows you to control the output appearance
using the format for the 'printf()' function in the C programming language. If you only want to change
the number of significant digits or specify exponential format type, use the '--digits' and
'--exponential' options. The '--output-format' option affords the greatest control of the output appear‐
ance, but requires at least rudimentary knowledge of the 'printf()' format syntax. See Invoking Units for
descriptions of these options.
Format Specification
The format specification recognized with the '--output-format' option is a subset of that for 'printf()'.
The format specification has the form '%'[flags][width]['.'precision]type; it must begin with '%', and
must end with a floating-point type specifier: 'g' or 'G' to specify the number of significant digits,
'e' or 'E' for scientific notation, and 'f' for fixed-point decimal. The ISO C99 standard added the 'F'
type for fixed-point decimal and the 'a' and 'A' types for hexadecimal floating point; these types are
allowed with compilers that support them. Type length modifiers (e.g., 'L' to indicate a long double)
are inapplicable and are not allowed.
The default format for 'units' is '%.8g'; for greater precision, you could specify '-o %.15g'. The 'g'
and 'G' format types use exponential format whenever the exponent would be less than -4, so the value
0.000013 displays as '1.3e-005'. These types also use exponential notation when the exponent is greater
than or equal to the precision, so with the default format, the value 5e7 displays as '50000000' and the
value 5e8 displays as '5e+008'. If you prefer fixed-point display, you might specify '-o %.8f'; however,
small numbers will display very few significant digits, and values less than 0.5e-8 will show nothing but
zeros.
The format specification may include one or more optional flags: '+', ' ' (space), '#', '-', or '0' (the
digit zero). The digit-grouping flag ''' is allowed with compilers that support it. Flags are followed
by an optional value for the minimum field width, and an optional precision specification that begins
with a period (e.g., '.6'). The field width includes the digits, decimal point, the exponent, thousands
separators (with the digit-grouping flag), and the sign if any of these are shown.
Flags
The '+' flag causes the output to have a sign ('+' or '-'). The space flag ' ' is similar to the '+'
flag, except that when the value is positive, it is prefixed with a space rather than a plus sign; this
flag is ignored if the '+' flag is also given. The '+' or ' ' flag could be useful if conversions might
include positive and negative results, and you wanted to align the decimal points in exponential nota‐
tion. The '#' flag causes the output value to contain a decimal point in all cases; by default, the out‐
put contains a decimal point only if there are digits (which can be trailing zeros) to the right of the
point. With the 'g' or 'G' types, the '#' flag also prevents the suppression of trailing zeros. The
digit-grouping flag ''' shows a thousands separator in digits to the left of the decimal point. This can
be useful when displaying large numbers in fixed-point decimal; for example, with the format '%f',
You have: mile
You want: microfurlong
* 8000000.000000
/ 0.000000
the magnitude of the first result may not be immediately obvious without counting the digits to the left
of the decimal point. If the thousands separator is the comma (','), the output with the format '%'f'
might be
You have: mile
You want: microfurlong
* 8,000,000.000000
/ 0.000000
making the magnitude readily apparent. Unfortunately, few compilers support the digit-grouping flag.
With the '-' flag, the output value is left aligned within the specified field width. If a field width
greater than needed to show the output value is specified, the '0' (zero) flag causes the output value to
be left padded with zeros until the specified field width is reached; for example, with the format
'%011.6f',
You have: troypound
You want: grain
* 5760.000000
/ 0000.000174
The '0' flag has no effect if the '-' (left align) flag is given.
Field Width
By default, the output value is left aligned and shown with the minimum width necessary for the specified
(or default) precision. If a field width greater than this is specified, the value shown is right
aligned, and padded on the left with enough spaces to provide the specified field width. A width speci‐
fication is typically used with fixed-point decimal to have columns of numbers align at the decimal
point; this arguably is less useful with 'units' than with long columnar output, but it may nonetheless
assist in quickly assessing the relative magnitudes of results. For example, with the format '%12.6f',
You have: km
You want: in
* 39370.078740
/ 0.000025
You have: km
You want: rod
* 198.838782
/ 0.005029
You have: km
You want: furlong
* 4.970970
/ 0.201168
Precision
The meaning of ``precision'' depends on the format type. With 'g' or 'G', it specifies the number of
significant digits (like the '--digits' option); with 'e', 'E', 'f', or 'F', it specifies the maximum
number of digits to be shown after the decimal point.
With the 'g' and 'G' format types, trailing zeros are suppressed, so the results may sometimes have fewer
digits than the specified precision (as indicated above, the '#' flag causes trailing zeros to be dis‐
played).
The default precision is 6, so '%g' is equivalent to '%.6g', and would show the output to six significant
digits. Similarly, '%e' or '%f' would show the output with six digits after the decimal point.
The C 'printf()' function allows a precision of arbitrary size, whether or not all of the digits are
meaningful. With most compilers, the maximum internal precision with 'units' is 15 decimal digits (or 13
hexadecimal digits). With the '--digits' option, you are limited to the maximum internal precision; with
the '--output-format' option, you may specify a precision greater than this, but it may not be meaning‐
ful. In some cases, specifying excess precision can result in rounding artifacts. For example, a pound
is exactly 7000 grains, but with the format '%.18g', the output might be
You have: pound
You want: grain
* 6999.9999999999991
/ 0.00014285714285714287
With the format '%.25g' you might get the following:
You have: 1/3
You want:
Definition: 0.333333333333333314829616256247
In this case the displayed value includes a series of digits that represent the underlying binary float‐
ing-point approximation to 1/3 but are not meaningful for the desired computation. In general, the
result with excess precision is system dependent. The precision affects only the display of numbers; if
a result relies on physical constants that are not known to the specified precision, the number of physi‐
cally meaningful digits may be less than the number of digits shown.
See the documentation for 'printf()' for more detailed descriptions of the format specification.
The '--output-format' option is incompatible with the '--exponential' or '--digits' options; if the for‐
mer is given in combination with either of the latter, the format is controlled by the last option given.
LOCALIZATION
Some units have different values in different locations. The localization feature accommodates this by
allowing a units data file to specify definitions that depend on the user's locale.
Locale
A locale is a subset of a user's environment that indicates the user's language and country, and some
attendant preferences, such as the formatting of dates. The 'units' program attempts to determine the
locale from the POSIX setlocale function; if this cannot be done, 'units' examines the environment vari‐
ables 'LC_CTYPE' and 'LANG'. On POSIX systems, a locale is of the form language'_'country, where lan‐
guage is the two-character code from ISO 639-1 and country is the two-character code from ISO 3166-1;
language is lower case and country is upper case. For example, the POSIX locale for the United Kingdom is
'en_GB'.
On systems running Microsoft Windows, the value returned by setlocale() is different from that on POSIX
systems; 'units' attempts to map the Windows value to a POSIX value by means of a table in the file
'locale.map' in the same directory, typically '/usr/local/share/units', as the default units data files.
The file includes entries for many combinations of language and country, and can be extended to include
other combinations. The 'locale.map' comprises two tab-separated columns; each entry is of the form
Windows-locale POSIX-locale
where POSIX-locale is as described above, and Windows-locale typically spells out both the language and
country. For example, the entry for the United States is
English_United States en_US
You can force 'units' to run in a desired locale by using the '-l' option.
In order to create unit definitions for a particular locale you begin a block of definitions in a unit
datafile with '!locale' followed by a locale name. The '!' must be the first character on the line.
The 'units' program reads the following definitions only if the current locale matches. You end the
block of localized units with '!endlocale'. Here is an example, which defines the British gallon.
!locale en_GB
gallon 4.54609 liter
!endlocale
Additional Localization
Sometimes the locale isn't sufficient to determine unit preferences. There could be regional prefer‐
ences, or a company could have specific preferences. Though probably uncommon, such differences could
arise with the choice of English customary units outside of English-speaking countries. To address this,
'units' allows specifying definitions that depend on environment variable settings. The environment
variables can be controled based on the current locale, or the user can set them to force a particular
group of definitions.
A conditional block of definitions in a units data file begins with either '!var' or '!varnot' following
by an environment variable name and then a space separated list of values. The leading '!' must appear
in the first column of a units data file, and the conditional block is terminated by '!endvar'. Defini‐
tions in blocks beginning with '!var' are executed only if the environment variable is exactly equal to
one of the listed values. Definitions in blocks beginning with '!varnot' are executed only if the envi‐
ronment variable does not equal any of the list values.
The inch has long been a customary measure of length in many places. The word comes from the latin uncia
meaning ``one twelfth,'' referring to its relationship with the foot. By the 20th century, the inch was
officially defined in English-speaking countries relative to the yard, but until 1959, the yard differed
slightly among those countries. In France the customary inch, which was displaced in 1799 by the meter,
had a different length based on a french foot. These customary definitions could be accommodated as fol‐
lows:
!var INCH_UNIT usa
yard 3600|3937 m
!endvar
!var INCH_UNIT canada
yard 0.9144 meter
!endvar
!var INCH_UNIT uk
yard 0.91439841 meter
!endvar
!var INCH_UNIT canada uk usa
foot 1|3 yard
inch 1|12 foot
!endvar
!var INCH_UNIT france
foot 144|443.296 m
inch 1|12 foot
line 1|12 inch
!endvar
!varnot INCH_UNIT usa uk france canada
!message Unknown value for INCH_UNIT
!endvar
When 'units' reads the above definitions it will check the environment variable 'INCH_UNIT' and load only
the definitions for the appropriate section. If 'INCH_UNIT' is unset or is not set to one of the four
values listed then 'units' will run the last block. In this case that block uses the '!message' command
to display a warning message. Alternatively that block could set default values.
In order to create default values that are overridden by user settings the data file can use the '!set'
command, which sets an environment variable only if it is not already set; these settings are only for
the current 'units' invocation and do not persist. So if the example above were preceded by '!set
INCH_UNIT france' then this would make 'france' the default value for 'INCH_UNIT'. If the user had set
the variable in the environment before invoking 'units', then 'units' would use the user's value.
To link these settings to the user's locale you combine the '!set' command with the '!locale' command.
If you wanted to combine the above example with suitable locales you could do by preceding the above def‐
inition with the following:
!locale en_US
!set INCH_UNIT usa
!endlocale
!locale en_GB
!set INCH_UNIT uk
!endlocale
!locale en_CA
!set INCH_UNIT canada
!endlocale
!locale fr_FR
!set INCH_UNIT france
!endlocale
!set INCH_UNIT france
These definitions set the overall default for 'INCH_UNIT' to 'france' and set default values for four
locales appropriately. The overall default setting comes last so that it only applies when 'INCH_UNIT'
was not set by one of the other commands or by the user.
If the variable given after '!var' or '!varnot' is undefined then 'units' prints an error message and
ignores the definitions that follow. Use '!set' to create defaults to prevent this situation from aris‐
ing. The '-c' option only checks the definitions that are active for the current environment and locale,
so when adding new definitions take care to check that all cases give rise to a well defined set of defi‐
nitions.
ENVIRONMENT VARIABLES
The 'units' program uses the following environment variables:
HOME Specifies the location of your home directory; it is used by 'units' to find a personal units data
file '.units'. On systems running Microsoft Windows, 'units' tries to determine your home direc‐
tory from the 'HOMEDRIVE' and 'HOMEPATH' environment variables if 'HOME' does not exist.
LC_CTYPE, LANG
Checked to determine the locale if 'units' cannot obtain it from the operating system. Sections
of the standard units data file are specific to certain locales.
MYUNITSFILE
Specifies your personal units data file. If this variable exists, 'units' uses its value rather
than searching your home directory for '.units'. The personal units file will not be loaded if
any data files are given using the '-f' option.
PAGER Specifies the pager to use for help and for displaying the conformable units. The help function
browses the units database and calls the pager using the '+n'n syntax for specifying a line num‐
ber. The default pager is 'more'; 'PAGER' can be used to specify alternatives such as 'less',
'pg', 'emacs', or 'vi'.
UNITS_ENGLISH
Set to either 'US' or 'GB' to choose United States or British volume definitions, overriding the
default from your locale.
UNITSFILE
Specifies the units data file to use (instead of the default). You can only specify a single
units data file using this environment variable. If units data files are given using the '-f'
option, the file specified by 'UNITSFILE' will be not be loaded unless the '-f' option is given
with the empty string ('units -f ""').
UNICODE SUPPORT
The standard units data file is in Unicode, using UTF-8 encoding. Most definitions use only ASCII char‐
acters (i.e., code points U+0000 through U+007F); definitions using non-ASCII characters appear in blocks
beginning with '!utf8' and ending with '!endutf8'.
When 'units' starts, it checks the locale to determine the character set. If 'units' is compiled with
Unicode support and if the character set is UTF-8, 'units' reads the UTF-8 definitions; otherwise these
definitions are ignored. When Unicode support is active, 'units' will check every line of all of the
units data files for invalid or non-printing UTF-8 sequences; if such sequences occur, 'units' ignores
the entire line. In addition to checking validity, 'units' determines the display width of non-ASCII
characters to ensure proper positioning of the pointer in some error messages and to align columns for
the 'search' and '?' commands.
At present, 'units' does not support Unicode under Microsoft Windows. The UTF-16 and UTF-32 encodings
are not supported on any systems.
If definitions that contain non-ASCII characters are added to a units data file, those definitions should
be enclosed within '!utf8' ... '!endutf8' to ensure that they are only loaded when Unicode support is
available. As usual, the '!' must appear as the first character on the line. As discussed in Units
Data Files, it's usually best to put such definitions in supplemental data files linked by an '!include'
command or in a personal units data file.
When Unicode support is not active, 'units' makes no assumptions about character encoding, except that
characters in the range 00-7F hexadecimal correspond to ASCII encoding. Non-ASCII characters are simply
sequences of bytes, and have no special meanings; for definitions in supplementary units data files, you
can use any encoding consistent with this assumption. For example, if you wish to use non-ASCII charac‐
ters in definitions when running 'units' under Windows, you can use a character set such as Windows
``ANSI'' (code page 1252 in the US and Western Europe). You can even use UTF-8, though some messages may
be improperly aligned, and 'units' will not detect invalid UTF-8 sequences. If you use UTF-8 encoding
when Unicode support is not active, you should place any definitions with non-ASCII characters outside
'!utf8' ... '!endutf8' blocks—otherwise, they will be ignored.
Typeset material other than code examples usually uses the Unicode minus (U+2212) rather than the ASCII
hyphen-minus operator (U+002D) used in 'units'; the figure dash (U+2012) and en dash (U+2013) are also
occasionally used. To allow such material to be copied and pasted for interactive use or in units data
files, 'units' converts these characters to U+002D before further processing. Because of this, none of
these characters can appear in unit names.
READLINE SUPPORT
If the 'readline' package has been compiled in, then when 'units' is used interactively, numerous command
line editing features are available. To check if your version of 'units' includes 'readline', invoke the
program with the '--version' option.
For complete information about 'readline', consult the documentation for the 'readline' package. Without
any configuration, 'units' will allow editing in the style of emacs. Of particular use with 'units' are
the completion commands.
If you type a few characters and then hit ESC followed by '?' then 'units' will display a list of all
the units that start with the characters typed. For example, if you type 'metr' and then request comple‐
tion, you will see something like this:
You have: metr
metre metriccup metrichorsepower metrictenth
metretes metricfifth metricounce metricton
metriccarat metricgrain metricquart metricyarncount
You have: metr
If there is a unique way to complete a unitname, you can hit the TAB key and 'units' will provide the
rest of the unit name. If 'units' beeps, it means that there is no unique completion. Pressing the TAB
key a second time will print the list of all completions.
UPDATING CURRENCY EXCHANGE RATES
The units program includes currency exchange rates and prices for some precious metals in the database.
Of course, these values change over time, sometimes very rapidly, and 'units' cannot provide real time
values. To update the exchange rates run the 'units_cur', which rewrites the files containing the cur‐
rency rates, typically '/usr/local/share/units/currency.units'. This program must be run with suitable
permissions to write the file. To keep the rates updated automatically, it could be run by a cron job on
a Unix-like system, or a similar scheduling program on a different system. Currency exchange rates are
taken from Time Genie (http://www.timegenie.com) and precious metals pricing from Packetizer (www.packe‐
tizer.com). These sites update once per day, so there is no benefit in running the update script more
often than daily. You can run 'units_cur' with a filename specified on the command line and it will
write the data to that file. If you give '-' for the file it will write to standard output.
DATABASE COMMAND SYNTAX
unit definition
Define a regular unit.
prefix- definition
Define a prefix.
funcname(var) units=[in-units,out-units] domain=[x1,x2] range=[y1,y2] definition(var) ; inverse(funcname)
Define a nonlinear unit or unit function. The three optional keywords 'units=', 'range=' and
'domain=' can appear in any order. The definition of the inverse is optional.
tabname[out-units] pair-list
Define a piecewise linear unit. The pair list gives the points on the table listed in ascending
order.
!endlocale
End a block of definitions beginning with '!locale'
!endutf8
End a block of definitions begun with '!utf8'
!endvar
End a block of definitions begun with '!var' or '!varnot'
!include file
Include the specified file.
!locale value
Load the following definitions only of the locale is set to value.
!message text
Display text when the database is read unless the quiet option ('-q') is enabled.
!set variable value
Sets the environment variable, variable, to the specified value only if it is not already set.
!unitlist alias definition
Define a unit list alias.
!utf8 Load the following definitions only if 'units' is running with UTF-8 enabled.
!var variable value-list
Load the following definitions only if the environment variable, variable is set to one of the
values listed on the space separated value list. If variable is not set then 'units' prints an
error message and ignores the following definitions.
!varnot variable value-list
Load the following definitions only if the environment variable, variable is not set to one of the
values listed on the space separated value list. If variable is not set then 'units' prints an
error message and ignores the following definitions.
GNU FREE DOCUMENTATION LICENSE
FILES
/usr/share/units/definitions.units — the standard units data file
AUTHOR
7 June 2013 UNITS(1)