NAME

       units - decimal and binary prefixes

DESCRIPTION

   Decimal prefixes
       The  SI  system  of  units uses prefixes that indicate powers of ten.  A kilometer is 1000
       meter, and a megawatt is 1000000 watt.  Below the standard prefixes.

              Prefix   Name    Value
              y        yocto   10^-24 = 0.000000000000000000000001
              z        zepto   10^-21 = 0.000000000000000000001
              a        atto    10^-18 = 0.000000000000000001
              f        femto   10^-15 = 0.000000000000001
              p        pico    10^-12 = 0.000000000001
              n        nano    10^-9  = 0.000000001
              µ        micro   10^-6  = 0.000001
              m        milli   10^-3  = 0.001
              c        centi   10^-2  = 0.01
              d        deci    10^-1  = 0.1
              da       deka    10^ 1  = 10
              h        hecto   10^ 2  = 100
              k        kilo    10^ 3  = 1000
              M        mega    10^ 6  = 1000000
              G        giga    10^ 9  = 1000000000
              T        tera    10^12  = 1000000000000
              P        peta    10^15  = 1000000000000000
              E        exa     10^18  = 1000000000000000000
              Z        zetta   10^21  = 1000000000000000000000
              Y        yotta   10^24  = 1000000000000000000000000

       The symbol for micro is the Greek letter mu, often written u in  an  ASCII  context  where
       this Greek letter is not available.  See also

              ⟨http://physics.nist.gov/cuu/Units/prefixes.html⟩

   Binary prefixes
       The binary prefixes resemble the decimal ones, but have an additional 'i' (and "Ki" starts
       with a capital 'K').  The names are formed by taking the first syllable of  the  names  of
       the decimal prefix with roughly the same size, followed by "bi" for "binary".

              Prefix   Name   Value
              Ki       kibi   2^10 = 1024
              Mi       mebi   2^20 = 1048576
              Gi       gibi   2^30 = 1073741824
              Ti       tebi   2^40 = 1099511627776
              Pi       pebi   2^50 = 1125899906842624
              Ei       exbi   2^60 = 1152921504606846976

       See also

       ⟨http://physics.nist.gov/cuu/Units/binary.html⟩

   Discussion
       Before  these  binary  prefixes  were  introduced,  it was fairly common to use k=1000 and
       K=1024, just like b=bit, B=byte.  Unfortunately, the M is capital already, and  cannot  be
       capitalized to indicate binary-ness.

       At  first  that  didn't matter too much, since memory modules and disks came in sizes that
       were powers of two, so everyone knew that in such contexts "kilobyte" and "megabyte" meant
       1024  and  1048576  bytes, respectively.  What originally was a sloppy use of the prefixes
       "kilo" and "mega" started to become regarded as the "real  true  meaning"  when  computers
       were involved.  But then disk technology changed, and disk sizes became arbitrary numbers.
       After a period of uncertainty all disk  manufacturers  settled  on  the  standard,  namely
       k=1000, M=1000 k, G=1000 M.

       The  situation was messy: in the 14k4 modems, k=1000; in the 1.44 MB diskettes, M=1024000;
       and so on.  In 1998 the IEC approved the standard that defines the binary  prefixes  given
       above, enabling people to be precise and unambiguous.

       Thus, today, MB = 1000000 B and MiB = 1048576 B.

       In  the  free software world programs are slowly being changed to conform.  When the Linux
       kernel boots and says

           hda: 120064896 sectors (61473 MB) w/2048KiB Cache

       the MB are megabytes and the KiB are kibibytes.

COLOPHON

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