man > units(7)

units(7)                          Miscellaneous Information 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
              q        quecto   10^-30 = 0.000000000000000000000000000001
              r        ronto    10^-27 = 0.000000000000000000000000001
              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
              R        ronna    10^27  = 1000000000000000000000000000
              Q        quetta   10^30  = 1000000000000000000000000000000

       The  symbol  for micro is the Greek letter mu, often written u in an ASCII context where this
       Greek letter is not available.

   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
              Zi       zebi   2^70 = 1180591620717411303424
              Yi       yobi   2^80 = 1208925819614629174706176

   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 conform.  When the Linux ker‐
       nel boots and says

           hda: 120064896 sectors (61473 MB) w/2048KiB Cache

       the MB are megabytes and the KiB are kibibytes.

SEE ALSO
       The International System of Units.

Linux man-pages 6.7                          2023-10-31                                     units(7)