units

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 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=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  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.



Linux                             2001-12-22                          UNITS(7)