Mbyte

A megabyte is a unit of measurement for computer storage, memory and information; while its exact definition varies, it is in theory equal to one million bytes. The symbol for megabyte is MB (note B for Byte, lowercase b would mean bit).

Three definitions for megabyte are being used:

1. 1 000 000 bytes or 10⁶ bytes - this is the definition used by telecommunications engineers and storage manufacturers among others. It is consistent with the SI prefix "mega" and is endorsed by international standards bodies.
2. 1 048 576 bytes - 1024², or 2²⁰. This definition is often used in computer science and computer programming, when talking about the size of files or computer memory. The reason is that computers use the binary numeral system internally.
3. 1 024 000 bytes - 1024 times 1000. This is an (erroneous) definition used by floppy disk manufacturers. See mebibyte.

The definitions of the kilobyte (either 1000 = 10³ or 1024 = 2¹⁰ bytes) and of the gigabyte (either 10⁹ or 2³⁰ bytes) have similar ambiguities.

To reduce the confusion and distinguish between meanings (1) and (2) above, the International Electrotechnical Commission (IEC), adopted an international standard in December 1998 which reserves the term megabyte for 10⁶ bytes and introduces the new term mebibyte (abbreviated as MiB) for 2²⁰ bytes. Similarly, the terms kibibyte (KiB, equal to 2¹⁰ bytes) and gibibyte (GiB, equal to 2³⁰ bytes) were introduced. This naming convention, while strongly endorsed by IEEE and CIPM, has not yet been widely accepted, and is simply ignored by most people.

Note the distinction between a megabyte (one million bytes) and a megabit (one million bits). A megabit is abbreviated as Mbit (preferably) or as Mb (when the byte is abbreviated by an upper case "B", which stands for Bel in SI). There are eight bits in one byte, so a megabyte (Mb) is eight times as large as a megabit (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 these contexts, one megabit is almost invariably defined as 10⁶ bits. In practice, the abbreviation Mb is frequently encountered as a mistaken notation for Mbit. In most cases, an examination of the context will indicate which unit of measure was intended.

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

The prefix "mega" comes from a Greek word meaning "large", and it was chosen because a million is a large number.

See also

• Binary prefix
• Integral data type.