units

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

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
              u        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 tak-
       ing 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  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
       This  page  is  part of release 4.15 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                          UNITS(7)
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