man7/units.7

   1 .\" Copyright (C) 2001 Andries Brouwer <aeb@cwi.nl>
   2 .\"
   3 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   4 .\"
   5 .TH UNITS 7 2020-08-13 "Linux man-pages (unreleased)"
   6 .SH NAME
   7 units \- decimal and binary prefixes
   8 .SH DESCRIPTION
   9 .SS Decimal prefixes
  10 The SI system of units uses prefixes that indicate powers of ten.
  11 A kilometer is 1000 meter, and a megawatt is 1000000 watt.
  12 Below the standard prefixes.
  13 .RS
  14 .TS
  15 l l l.
  16 Prefix  Name    Value
  17 y       yocto   10^\-24 = 0.000000000000000000000001
  18 z       zepto   10^\-21 = 0.000000000000000000001
  19 a       atto    10^\-18 = 0.000000000000000001
  20 f       femto   10^\-15 = 0.000000000000001
  21 p       pico    10^\-12 = 0.000000000001
  22 n       nano    10^\-9  = 0.000000001
  23 \(mc    micro   10^\-6  = 0.000001
  24 m       milli   10^\-3  = 0.001
  25 c       centi   10^\-2  = 0.01
  26 d       deci    10^\-1  = 0.1
  27 da      deka    10^ 1  = 10
  28 h       hecto   10^ 2  = 100
  29 k       kilo    10^ 3  = 1000
  30 M       mega    10^ 6  = 1000000
  31 G       giga    10^ 9  = 1000000000
  32 T       tera    10^12  = 1000000000000
  33 P       peta    10^15  = 1000000000000000
  34 E       exa     10^18  = 1000000000000000000
  35 Z       zetta   10^21  = 1000000000000000000000
  36 Y       yotta   10^24  = 1000000000000000000000000
  37 .TE
  38 .RE
  39 .PP
  40 The symbol for micro is the Greek letter mu, often written u
  41 in an ASCII context where this Greek letter is not available.
  42 See also
  43 .PP
  44 .RS
  45 .UR http://physics.nist.gov\:/cuu\:/Units\:/prefixes.html
  46 .UE
  47 .RE
  48 .SS Binary prefixes
  49 The binary prefixes resemble the decimal ones,
  50 but have an additional \(aqi\(aq
  51 (and "Ki" starts with a capital \(aqK\(aq).
  52 The names are formed by taking the
  53 first syllable of the names of the decimal prefix with roughly the same
  54 size, followed by "bi" for "binary".
  55 .RS
  56 .TS
  57 l l l.
  58 Prefix  Name    Value
  59 Ki      kibi    2^10 = 1024
  60 Mi      mebi    2^20 = 1048576
  61 Gi      gibi    2^30 = 1073741824
  62 Ti      tebi    2^40 = 1099511627776
  63 Pi      pebi    2^50 = 1125899906842624
  64 Ei      exbi    2^60 = 1152921504606846976
  65 .TE
  66 .RE
  67 .PP
  68 See also
  69 .PP
  70 .UR http://physics.nist.gov\:/cuu\:/Units\:/binary.html
  71 .UE
  72 .SS Discussion
  73 Before these binary prefixes were introduced, it was fairly
  74 common to use k=1000 and K=1024, just like b=bit, B=byte.
  75 Unfortunately, the M is capital already, and cannot be
  76 capitalized to indicate binary-ness.
  77 .PP
  78 At first that didn't matter too much, since memory modules
  79 and disks came in sizes that were powers of two, so everyone
  80 knew that in such contexts "kilobyte" and "megabyte" meant
  81 1024 and 1048576 bytes, respectively.
  82 What originally was a
  83 sloppy use of the prefixes "kilo" and "mega" started to become
  84 regarded as the "real true meaning" when computers were involved.
  85 But then disk technology changed, and disk sizes became arbitrary numbers.
  86 After a period of uncertainty all disk manufacturers settled on the
  87 standard, namely k=1000, M=1000\ k, G=1000\ M.
  88 .PP
  89 The situation was messy: in the 14k4 modems, k=1000; in the 1.44\ MB
  90 .\" also common: 14.4k modem
  91 diskettes, M=1024000; and so on.
  92 In 1998 the IEC approved the standard
  93 that defines the binary prefixes given above, enabling people
  94 to be precise and unambiguous.
  95 .PP
  96 Thus, today, MB = 1000000\ B and MiB = 1048576\ B.
  97 .PP
  98 In the free software world programs are slowly
  99 being changed to conform.
 100 When the Linux kernel boots and says
 101 .PP
 102 .in +4n
 103 .EX
 104 hda: 120064896 sectors (61473 MB) w/2048KiB Cache
 105 .EE
 106 .in
 107 .PP
 108 the MB are megabytes and the KiB are kibibytes.
 109 .SH SEE ALSO
 110 .UR https://www.bipm.org/documents/20126/41483022/SI-Brochure-9.pdf
 111 The International System of Units
 112 .UE .