The mebibyte is a multiple of the unit byte for digital information. The binary prefix mebi means 220, therefore 1 mebibyte is 1048576bytes. The unit symbol for the mebibyte is MiB. The unit was established by the International Electrotechnical Commission (IEC) in 2000. It was designed to replace the megabyte used in some computer science contexts to mean 220 bytes, which is similar to the SI definition of the prefix mega (106) but conflicts with it.
The unit has been accepted for use by all major standards organizations, but has seen little usage in the computer industry. The unit megabyte (symbol MB), formally meaning 1000000bytes, is still commonly used in place of this unit.
|SI decimal prefixes||Binary
|IEC binary prefixes|
|kilobyte (kB)||103||210||kibibyte (KiB)||210|
|megabyte (MB)||106||220||mebibyte (MiB)||220|
|gigabyte (GB)||109||230||gibibyte (GiB)||230|
|terabyte (TB)||1012||240||tebibyte (TiB)||240|
|petabyte (PB)||1015||250||pebibyte (PiB)||250|
|exabyte (EB)||1018||260||exbibyte (EiB)||260|
|zettabyte (ZB)||1021||270||zebibyte (ZiB)||270|
|yottabyte (YB)||1024||280||yobibyte (YiB)||280|
|See also: Multiples of bits · Orders of magnitude of data|
The prefix mebi is a binary prefix derived from the words mega and binary, indicating its origin in the closeness in value to the SI prefix mega. One mebibyte (MiB) is 220 (i.e., 1024 x 1024) bytes, or 1048576bytes. One MiB differs from one megabyte (MB), which means 106 (i.e. 1000 x 1000 = 1000000) bytes.
Mebibyte is not commonly used. Instead, megabyte is often used to mean 1000 x 1000, 1024 x 1024 or even 1024 x 1000 by disk manufacturers. Such usage can be confusing and inconsistent, since operating systems using the nonstandards-based method report lower capacities for hard disks than advertised by manufacturers. Most operating systems compute file size in Mebibytes, but report the number as MB. For example, all versions of Microsoft Windows operating system shows a file of 220 bytes as "1.00 MB" in its file properties dialog, while showing a file of 106 (1000000) bytes as 976 kB. All versions of Apple's operating systems had the same behaviour, until Mac OS X version 10.6, which now uses Megabytes for all file and disk sizes, so it reports a 106 byte file as 1 MB.
Confusion in the meaning of megabyte was evident for many years. For instance, the 1.44 MB floppy disk's storage capacity was calculated using 1024000bytes per "MB" (i.e. 1.44×1024×1000), rather than 1.47 MB (1.47×1000×1000) or 1.40 MiB (1.40×1024×1024).
The mebi- prefix was defined by the International Electrotechnical Commission (IEC) in December 1998. Its use (and related units) is presently endorsed by the Institute of Electrical and Electronics Engineers (IEEE) and the International Committee for Weights and Measures (CIPM) in contexts where use of a binary prefix makes sense.
See also 
- International Electrotechnical Commission (2010-01). "IEC 60050 - International Electrotechnical Vocabulary - Details for IEV number 112-01-27". Retrieved 2011-06-19.
- Upgrading and Repairing PCs, Scott Mueller, Pg. 596, ISBN 0-7897-2974-1
- The silicon web: physics for the Internet age, Michael G. Raymer, Pg. 40, ISBN 978-1-4398-0311-0
- Knuth: Recent News. Cs-staff.stanford.edu. Retrieved on 2011-01-07.
- Atwood, Jeff. (2007-09-10) Gigabyte: Decimal vs. Binary. Coding Horror. Retrieved on 2011-01-07.
- "Definition of NIST binary". Ziff-Davis. 2010. Retrieved 2010-07-31.
- "How Mac OS X reports drive capacity". Apple Inc. 2009-08-27. Retrieved 2009-10-16.
- David Pogue (2011), Mac OS X Lion: The Missing Manual Missing Manual, Oreilly Series, O'Reilly Media, pp. 473–474, ISBN 978-1-4493-9749-4