From Wikipedia, the free encyclopedia
A megabyte is a unit of information or computer storage equal to approximately one million bytes. It is commonly abbreviated MB. Mb is used for megabits.
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, of which only 1,457,664 bytes are usable on a Windows computer), and the "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 "typical" sized photograph with reasonably good quality
- Roughly, a minute of near CD-quality MP3 compressed music (at 128 kbit/s)
- Approximately 100 pages of single-spaced 12 point font text in Microsoft Word.
- Approximately 3 seconds of DVD-quality video
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.
Note that computer memory is addressed in base 2, due to its design, so memory size is always a power of two. It is thus convenient to measure in binary units. Other computer measurements, like storage hardware size, data transfer rates (despite the fact that hardware storage, and data transfer rates are both directly related to storing or moving chunks of data in base 2 computer memory), clock speeds, operations per second, and so on do not have an inherent base, and are usually measured in decimal units. Consumers who are (initially) unaware of varying meanings of the abbreviations often feel shortchanged when they discover the difference, and claim that manufacturers of drives and data transfer devices are using the decimal measurements in an intentionally misleading way to inflate their numbers, though these measurements are the norm in all fields other than computer memory and storage.
For instance, if a hard drive is said by a vendor to store 140 GB of data, the disk can store 140×109 bytes. Generally, operating systems allocate and report disk and files sizes in binary units, and present them using abbreviations (e.g GB, MB, KB) also used by the decimal system, so this drive would be reported as "130 GB" (actually 130.38 GiB). (Furthermore, the drive wouldn't be able to store files with a total filesize of 130.38 GiB, either, due to filesystem overhead. See Partition (computing).)
Several legal disputes have been waged over the confusion. See Binary prefix - Legal disputes.
- binary prefix
- orders of magnitude (data)
- Historical Notes About The Cost Of Hard Drive Storage Space
- the megabyte (established definition in Networking and Storage industries; from whatis.com)
- International Electrotechnical Commission definitions