1UNITS(7) Linux Programmer's Manual UNITS(7)
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6 units, kilo, kibi, mega, mebi, giga, gibi - decimal and binary prefixes
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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.
13 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
36 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
39 http://physics.nist.gov/cuu/Units/prefixes.html
41 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".
47 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
56 See also
58 http://physics.nist.gov/cuu/Units/binary.html
60 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.
65 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.
74 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.
79 Thus, today, MB = 1000000B and MiB = 1048576B.
81 In the free software world programs are slowly being changed to con‐
83 form. When the Linux kernel boots and says
84 hda: 120064896 sectors (61473 MB) w/2048KiB Cache
86 the MB are megabytes and the KiB are kibibytes.
88Linux 2001-12-22 UNITS(7)