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



NAME
       units, kilo, kibi, mega, mebi, giga, gibi - 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  avail-
       able.  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=1000k, G=1000M.

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

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

       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
       This  page  is  part of release 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2001-12-22                                                   UNITS(7)