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A megabyte is a unit of measurement in computers of approximately one million bytes. The abbreviation for megabyte is MB.

Because of irregularities in definition and usage of the kilobyte, the exact number could be any one of the following:

  1. 1 000 000 bytes or 106 - this is the definition used by telecommunications engineers and storage manufacturers among others. It is consistent with the SI prefix "mega".
  2. 1 048 576 bytes - 1024 times 1024, or 220. This definition is often used in computer science and computer programming.
  3. 1 024 000 bytes - 1024 times 1000. This is an (erroneous) definition used by floppy disk manufacturers

See integral data type.

A gigabyte is approximately 1000 or 1024 megabytes. Like the megabyte, its definition is variable.

To reduce the confusion and distinguish between meaning (1) and (2) above, the International Electrotechnical Commission (IEC), a standards body, in 1997 proposed short unions of the International System of Units (SI) prefixes with the word "binary" whenever powers of two rather than powers of ten are used. Thus meaning (2) would be called a mebibyte, abbreviated as MiB, and 1024 mebibytes would be 1 gibibyte, abbreviated as GiB. This naming convention has not yet been widely accepted.

Note the distinction between a megabyte (about one million bytes) and a megabit: (about one million bits). A megabit is abbreviated as Mb with a lower case "b" or Mbit. There are eight bits in one byte, so a megabyte (MB) is eight times as large as a megabit (Mb or Mbit). Megabits are often used in applications where a serial bitstream is the item of interest, particularly in communications and in specifying the internal data rate of a computer hard drive. In practice, the abbreviation Mb is frequently encountered as a mistaken notation for MB. Mb is also often used in print advertising, in order to subtly exaggerate the capacity or transfer rate of some product, i.e. Our disks hold 80 Mb; theirs only hold 10 MB. In most cases, an examination of the context will indicate which unit of measure was intended.

Similarly, a Gb or Gbit is a gigabit and a kb or kbit is a kilobit: these units too are often written in error when using the "b".

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