# units

## 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 kernel boots and says
    
               hda: 120064896 sectors (61473 MB) w/2048KiB Cache
    
           the MB are megabytes and the KiB are kibibytes.
    


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