A megabyte (derived from the SI prefix mega-) is a unit of information or computer storage equal to one million bytes. It is commonly abbreviated MB in writing (Mb is also sometimes seen) and meg in writing or speech.

Three definitions are in common use as of 2005:

1. 1,000,000 bytes (1000², 10⁶): This definition is used in most networking contexts and discussions of storage hardware, including hard drives and DVDs. It is consistent with the SI prefix and most other uses of the prefix in computing.
2. 1,024,000 bytes (1,024×1,000): This definition is used in a small number of storage contexts, most notably the "1.44 MB" (1,474,560 bytes) "3.5-inch" (actually 90 mm) high-density floppy disk.
3. 1,048,576 bytes (1024², 2²⁰): This definition is used for nearly all discussions of computer memory (most easily manufactured in power-of-two capacities) and CDs. As of 2005, most software uses this definition to express storage capacity. This quantity may be referred to unambiguously as a mebibyte (see binary prefixes).

kilobyte << megabyte << gigabyte

Megabytes in use

As of 2005, the RAM capacity of most personal computers is measured in megabytes.

Some rules of thumb: A megabyte stores roughly one book, or a hundred small images, or one or two very large ones, or roughly a minute of encoded music.