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

6       units - 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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14              Prefix   Name    Value
15              y        yocto   10^-24 = 0.000000000000000000000001
16              z        zepto   10^-21 = 0.000000000000000000001
17              a        atto    10^-18 = 0.000000000000000001
18              f        femto   10^-15 = 0.000000000000001
19              p        pico    10^-12 = 0.000000000001
20              n        nano    10^-9  = 0.000000001
21              µ        micro   10^-6  = 0.000001
22              m        milli   10^-3  = 0.001
23              c        centi   10^-2  = 0.01
24              d        deci    10^-1  = 0.1
25              da       deka    10^ 1  = 10
26              h        hecto   10^ 2  = 100
27              k        kilo    10^ 3  = 1000
28              M        mega    10^ 6  = 1000000
29              G        giga    10^ 9  = 1000000000
30              T        tera    10^12  = 1000000000000
31              P        peta    10^15  = 1000000000000000
32              E        exa     10^18  = 1000000000000000000
33              Z        zetta   10^21  = 1000000000000000000000
34              Y        yotta   10^24  = 1000000000000000000000000
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36       The  symbol  for  micro  is  the Greek letter mu, often written u in an
37       ASCII context where this Greek letter is not available.  See also
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39http://physics.nist.gov/cuu/Units/prefixes.html
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41   Binary prefixes
42       The binary prefixes resemble the decimal ones, but have  an  additional
43       'i' (and "Ki" starts with a capital 'K').  The names are formed by tak‐
44       ing the first syllable of the names of the decimal prefix with  roughly
45       the same size, followed by "bi" for "binary".
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47              Prefix   Name   Value
48              Ki       kibi   2^10 = 1024
49              Mi       mebi   2^20 = 1048576
50              Gi       gibi   2^30 = 1073741824
51              Ti       tebi   2^40 = 1099511627776
52              Pi       pebi   2^50 = 1125899906842624
53              Ei       exbi   2^60 = 1152921504606846976
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55       See also
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57http://physics.nist.gov/cuu/Units/binary.html
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59   Discussion
60       Before  these  binary prefixes were introduced, it was fairly common to
61       use k=1000 and K=1024, just like b=bit, B=byte.  Unfortunately,  the  M
62       is capital already, and cannot be capitalized to indicate binary-ness.
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64       At  first  that  didn't matter too much, since memory modules and disks
65       came in sizes that were powers of two, so everyone knew  that  in  such
66       contexts  "kilobyte"  and  "megabyte"  meant  1024  and  1048576 bytes,
67       respectively.  What originally was a sloppy use of the prefixes  "kilo"
68       and  "mega"  started to become regarded as the "real true meaning" when
69       computers were involved.  But then disk technology  changed,  and  disk
70       sizes became arbitrary numbers.  After a period of uncertainty all disk
71       manufacturers  settled  on  the  standard,  namely  k=1000,   M=1000 k,
72       G=1000 M.
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74       The  situation  was  messy:  in the 14k4 modems, k=1000; in the 1.44 MB
75       diskettes, M=1024000; and so on.  In 1998 the IEC approved the standard
76       that  defines  the  binary  prefixes given above, enabling people to be
77       precise and unambiguous.
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79       Thus, today, MB = 1000000 B and MiB = 1048576 B.
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81       In the free software world programs are slowly being  changed  to  con‐
82       form.  When the Linux kernel boots and says
83
84           hda: 120064896 sectors (61473 MB) w/2048KiB Cache
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86       the MB are megabytes and the KiB are kibibytes.
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COLOPHON

89       This  page  is  part of release 4.16 of the Linux man-pages project.  A
90       description of the project, information about reporting bugs,  and  the
91       latest     version     of     this    page,    can    be    found    at
92       https://www.kernel.org/doc/man-pages/.
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96Linux                             2017-09-15                          UNITS(7)
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