units (7)
Leading comments
Copyright (C) 2001 Andries Brouwer <aeb@cwi.nl> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the L...
NAME
units, kilo, kibi, mega, mebi, giga, gibi  decimal and binary prefixesDESCRIPTION
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 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 mc 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
The symbol for micro is the Greek letter mu, often written u in an ASCII context where this Greek letter is not available. See also
Binary prefixes
The binary prefixes resemble the decimal ones, but have an additional aqiaq (and "Ki" starts with a capital aqKaq). 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
See also
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 binaryness.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=1000k, G=1000M.
The situation was messy: in the 14k4 modems, k=1000; in the 1.44MB 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 = 1000000B and MiB = 1048576B.
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.