units(7) — Linux manual page


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

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       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

              ⟨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

              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-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.

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Linux                          2020-08-13                       UNITS(7)