Megabyte

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This article is about a unit of data. For the ReBoot character, see Megabyte (ReBoot).

Template:Quantities of bytes The megabyte is a unit of information or computer storage equal to either 106 (1,000,000) bytes or 220 (1,048,576) bytes, depending on context. In rare cases, it is used to mean 1000×1024 (1,024,000) bytes. It is commonly abbreviated as Mbyte or MB (compare Mb, for the megabit). The term megabyte was coined in 1970.[1]

Three definitions for 1 MB 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 - 1024 times 1024, 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.

Contents

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) is roughly equal to 1000 (103), roughly corresponding SI multiples began to be used as approximate binary multiples. By the end of 2007, standards and government authorities including IEC, IEEE, EU, and NIST, had addressed this ambiguity by promulgating standards requiring the use of megabyte to describe strictly 10002 bytes and "mebibyte" to describe 10242 bytes. This is reflected in an increasing number of software projects, but most file managers still show file sizes in "megabytes" ("MB") in the binary sense (10242 bytes). The term remains ambiguous and it can follow any one of the following common definitions:

  1. 1,000,000 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, 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.
  2. 1,048,576 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).
  3. 1,024,000 bytes (1000×1024): This is used to describe the formatted capacity of USB flash drives[2] and the "1.44 MB" 3.5 inch HD floppy disk, which actually has a capacity of 1,474,560 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).

Additional explanations

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 meaning (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. These naming convention, while strongly endorsed by IEEE and CIPM, have not yet been widely accepted, and are simply ignored by most people.

Note the distinction between a megabyte (about one million bytes) and a megabit (about one million bits). A megabit is abbreviated as Mbit (preferably) or as Mb with a lower case "b". 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 these contexts, one megabit is almost invariably defined as 106 bits. In practice, the abbreviation Mb is frequently encountered as a mistaken notation for 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".

See also

External links

Template:Computer Storage Volumes

References

  1. ^ "Megabyte."Webster's Ninth New Collegiate Dictionary. 9th ed. 1983.
  2. ^ SanDisk USB Flash Drive "Note: 1 megabyte (MB) = 1 million bytes; 1 gigabyte (GB) = 1 billion bytes."
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