GENWiki

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
                     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

 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

 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 4.16 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)