megabyte

The megabyte is an SI-multiple (see prefix mega-) of the unit byte for digital information storage or transmission and is equal to 10⁶ (1000000) bytes. However, due to historical usage in computer-related fields it is still often used to represent 2²⁰ (1024×1024 or 1048576) bytes. In rare cases, it is used to mean 1000×1024 (1024000) bytes. It is commonly abbreviated as Mbyte or MB (compare Mb, for the megabit).

Definition

The term "megabyte" is ambiguous because it is commonly used to mean either 1000² bytes or 1024² bytes. The confusion originated as compromise technical jargon for the byte multiples that needed to be expressed by the powers of 2 but lacked a convenient name. As 1024 (2¹⁰) approximates 1000 (10³), roughly corresponding SI multiples began to be used for binary multiples. By the end of 2007, standards and government authorities including IEC, IEEE, EU, and NIST, had addressed this ambiguity by promulgating standards for binary prefixes and requiring the use of megabyte to strictly denote 1000² bytes and mebibyte to denote 1024² bytes. This is reflected in an increasing number of software projects using the new definitions, but some file managers still show file sizes using the binary interpretation (1024² bytes). The term remains ambiguous and it can follow any one of the following common definitions:

1. 1000000 bytes (1000², 10⁶): This is the definition recommended by the International System of Units (SI) and the International Electrotechnical Commission IEC. This definition is used in networking contexts and most storage media, particularly hard drives, Flash-based storage^[1], and DVDs, and is also consistent with the other uses of the SI prefix in computing, such as CPU clock speeds or measures of performance. The Mac OS X 10.6 file manager is a notable example of this usage in software. Since Snow Leopard, file sizes are reported in decimal units.^[2]
2. 1048576 bytes (1024², 2²⁰): This definition is most commonly used in reference to computer memory, but most software that display file size or drive capacity, including file managers also use this definition. See Consumer confusion (in the "gigabyte" article). The Microsoft Windows file manager is a notable example of this usage in software.
3. 1024000 bytes (1000×1024): This is used to describe the formatted capacity of the "1.44 MB" 3.5 inch HD floppy disk, which actually has a capacity of 1474560 bytes.

Megabyte examples

1.44 MB floppy disks can store 1,474,560 bytes of data. MB in this context means 1,000×1,024 bytes.

Depending on compression methods and file format, a megabyte of data can roughly be:

• a 1024×1024 pixel bitmap image with 256 colors (8 bpp color depth).
• 1 minute of 128 kbit/s MP3 compressed music.
• 6 seconds of uncompressed CD audio.
• a typical book volume in text format (500 pages × 2000 characters per page).

See also

• Megabit
• Orders of magnitude (data)

References

External links

• 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
• IEC prefixes and symbols for binary multiples