units(7) — Linux manual page


units(7)            Miscellaneous Information 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
              q        quecto   10^-30 = 0.000000000000000000000000000001
              r        ronto    10^-27 = 0.000000000000000000000000001
              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
              R        ronna    10^27  = 1000000000000000000000000000
              Q        quetta   10^30  = 1000000000000000000000000000000

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

   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
              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
              Zi       zebi   2^70 = 1180591620717411303424
              Yi       yobi   2^80 = 1208925819614629174706176

       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.

SEE ALSO         top

       The International System of Units 

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02                       units(7)