units(7) — Linux manual page


UNITS(7)                  Linux Programmer's Manual                 UNITS(7)

NAME         top

       units - decimal and binary prefixes

DESCRIPTION         top

   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.htmlBinary 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

       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-

       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 mean‐
       ing" when computers were involved.  But then disk technology changed,
       and disk sizes became arbitrary numbers.  After a period of uncer‐
       tainty 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 stan‐
       dard 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 con‐
       form.  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         top

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Linux                            2020-08-13                         UNITS(7)