Megabyte
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A megabyte is a unit of information or computer storage equal to approximately one million bytes. Megabyte is commonly abbreviated as MB. Mb is used for megabits.
Definition
Because of inconsistencies in the use of SI-derived prefixes such as kilo- and mega-, the exact number can be any one of the following:
- 1,048,576 bytes (1,0242, 220): This definition is used for nearly all discussions of computer memory (as computer addresses are naturally powers of two, making it efficient to manufacture memory in power-of-two capacities) and file storage. As of 2005, most software uses this definition to express storage capacity (e.g. file size).
- 1,000,000 bytes (1,0002, 106): This is the definition recommended by SI and IEC. It is used primarily in networking contexts and most storage media, particularly hard drives and DVDs. This definition of 'mega-' as a SI prefix is consistent with the other SI prefixes, and with many other uses of the prefix in computing, such as CPU clock speeds or measures of performance.
- 1,024,000 bytes (1,024×1,000): This definition occurs rarely. It was used in a small number of storage contexts, most notably the "1.44 MB" (actually 1,474,560 bytes) "3.5-inch" (actually 90 mm) high-density floppy diskette.
Several attempts to resolve these inconsistencies have been proposed, most notably the standard created by the IEC to redefine 1,048,576 bytes as a "mebibyte". This standard has been adopted by several organizations, such as the NIST and IEEE. See Binary prefix for more information.
Megabytes in use
Usually, the storage capacities of hardware devices such as hard drives are given as 1 megabyte = 1,000,000 bytes (especially for items made in Japan, as Japan has standardized on the metric definition). Other capacities, such as for the RAM capacity of most personal computers, are given as 1 megabyte = 1,048,576 bytes (known unambiguously as a "mebibyte").
Depending on compression methods and file format, a megabyte of data can roughly hold:
- one larger book (excluding images),
- one "typical" sized photograph with reasonably good quality
- a hundred small images, or
- roughly a minute of compressed music.
A digital photograph produced by a typical digital camera in 2005 might be 1–4 MB depending on the camera's image resolution and level of compression used.
Until the introduction of hard drives with a capacity of one gigabyte or more, the capacity of hard drives was measured in megabytes.