1UNITS(7)                   Linux Programmer's Manual                  UNITS(7)
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NAME

6       units, kilo, kibi, mega, mebi, giga, gibi - decimal and binary prefixes
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DESCRIPTION

9   Decimal prefixes
10       The  SI  system  of units uses prefixes that indicate powers of ten.  A
11       kilometer is 1000 meter, and a megawatt is  1000000  watt.   Below  the
12       standard prefixes.
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15              Prefix   Name    Value
16              y        yocto   10^-24 = 0.000000000000000000000001
17              z        zepto   10^-21 = 0.000000000000000000001
18              a        atto    10^-18 = 0.000000000000000001
19              f        femto   10^-15 = 0.000000000000001
20              p        pico    10^-12 = 0.000000000001
21              n        nano    10^-9  = 0.000000001
22              u        micro   10^-6  = 0.000001
23              m        milli   10^-3  = 0.001
24              c        centi   10^-2  = 0.01
25              d        deci    10^-1  = 0.1
26              da       deka    10^ 1  = 10
27              h        hecto   10^ 2  = 100
28              k        kilo    10^ 3  = 1000
29              M        mega    10^ 6  = 1000000
30              G        giga    10^ 9  = 1000000000
31              T        tera    10^12  = 1000000000000
32              P        peta    10^15  = 1000000000000000
33              E        exa     10^18  = 1000000000000000000
34              Z        zetta   10^21  = 1000000000000000000000
35              Y        yotta   10^24  = 1000000000000000000000000
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37       The  symbol  for  micro  is  the Greek letter mu, often written u in an
38       ASCII context where this Greek letter is not available.  See also
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40              http://physics.nist.gov/cuu/Units/prefixes.html
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42   Binary prefixes
43       The binary prefixes resemble the decimal ones, but have  an  additional
44       'i'  (and "Ki" starts with a capital 'K'). The names are formed by tak‐
45       ing the first syllable of the names of the decimal prefix with  roughly
46       the same size, followed by "bi" for "binary".
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49              Prefix   Name   Value
50              Ki       kibi   2^10 = 1024
51              Mi       mebi   2^20 = 1048576
52              Gi       gibi   2^30 = 1073741824
53              Ti       tebi   2^40 = 1099511627776
54              Pi       pebi   2^50 = 1125899906842624
55              Ei       exbi   2^60 = 1152921504606846976
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57       See also
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59              http://physics.nist.gov/cuu/Units/binary.html
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61   Discussion
62       Before  these  binary prefixes were introduced, it was fairly common to
63       use k=1000 and K=1024, just like b=bit, B=byte.  Unfortunately,  the  M
64       is capital already, and cannot be capitalized to indicate binary-ness.
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66       At  first  that  didn't matter too much, since memory modules and disks
67       came in sizes that were powers of two, so everyone knew  that  in  such
68       contexts  "kilobyte"  and  "megabyte"  meant  1024  and  1048576 bytes,
69       respectively. What originally was a sloppy use of the  prefixes  "kilo"
70       and  "mega"  started to become regarded as the "real true meaning" when
71       computers were involved.  But then disk technology  changed,  and  disk
72       sizes became arbitrary numbers.  After a period of uncertainty all disk
73       manufacturers settled on the standard, namely k=1000, M=1000k, G=1000M.
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75       The situation was messy: in the 14k4  modems,  k=1000;  in  the  1.44MB
76       diskettes,  M=1024000;  etc. In 1998 the IEC approved the standard that
77       defines the binary prefixes given above, enabling people to be  precise
78       and unambiguous.
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80       Thus, today, MB = 1000000B and MiB = 1048576B.
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82       In  the  free  software world programs are slowly being changed to con‐
83       form. When the Linux kernel boots and says
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85              hda: 120064896 sectors (61473 MB) w/2048KiB Cache
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87       the MB are megabytes and the KiB are kibibytes.
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91Linux                             2001-12-22                          UNITS(7)
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