Mebibyte

The mebibyte is a multiple of the unit byte for digital information. The binary prefix mebi means 2²⁰, therefore 1 mebibyte is 1,048,576 bytes. The unit symbol for the mebibyte is MiB.^[1] 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 but conflicts with the SI definition of the prefix mega (10⁶).

It has been accepted for use by all major standards organizations, but it has seen little real world usage in the computer industry.^[2][3][4][5] The term megabyte and abbreviation MB are more commonly used for this unit, though they may be ambiguous with 1,000,000 bytes.

Definition

1 MiB = 2²⁰ bytes = 1024 kibibytes = 1048576bytes

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,^[6] or 1048576bytes. One MiB differs from one megabyte (MB), which means 10⁶ (i.e. 1000 x 1000 = 1000000bytes).

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 will report lower capacities for hard disks than advertised by manufacturers in the labels. Most operating systems use Mebibytes for file sizes. 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.^[7][8]

History

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.

In The Art of Computer Programming, Donald Knuth proposed that the mebibyte be called a large megabyte (abbreviated MMB).

See also

• Megabyte
• SI prefix
• IEEE 1541
• Orders of magnitude (data)

References