Megabyte

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

The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: 1048576 bytes (2²⁰) generally for computer memory;^[1][2] and one million bytes (10⁶, see prefix mega-) generally for computer storage.^[1][3] The IEEE Standards Board has decided that "Mega will mean 1 000 000", with exceptions allowed for the base-two meaning.^[3] In rare cases, it is used to mean 1000×1024 (1024000) bytes.^[3] It is commonly abbreviated as Mbyte or MB (compare Mb, for the megabit).

v · d · e Multiples of bytes
SI decimal prefixes		Binary usage	IEC binary prefixes
Name (Symbol)	Value		Name (Symbol)	Value
kilobyte (kB)	103	210	kibibyte (KiB)	210
megabyte (MB)	106	220	mebibyte (MiB)	220
gigabyte (GB)	109	230	gibibyte (GiB)	230
terabyte (TB)	1012	240	tebibyte (TiB)	240
petabyte (PB)	1015	250	pebibyte (PiB)	250
exabyte (EB)	1018	260	exbibyte (EiB)	260
zettabyte (ZB)	1021	270	zebibyte (ZiB)	270
yottabyte (YB)	1024	280	yobibyte (YiB)	280
See also: Multiples of bits · Orders of magnitude of data

Definition

The term "megabyte" is commonly used to mean either 1000² bytes or 1024² bytes. This 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 proposed 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:

• 1000000 bytes (1000², 10⁶): This is the definition recommended by the International System of Units (SI) and the International Electrotechnical Commission IEC.^[3] This definition is used in networking contexts and most storage media, particularly hard drives, Flash-based storage,^[4] 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.^[5]
• 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). This definition is synonymous with the unambiguous IEC standard name mebibyte, however as of 2011 adoption of the standard has not been widespread.
• 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.

RAM usage: In modern computer systems, blocks of main working memory (RAM) invariably come in sizes that are some-power-of-2 bytes, so for this reason, the binary M-means-1024×1024-system will almost always be used when talking about blocks of RAM. Example: "1 MB of RAM" =1×1024×1024 bytes (1048576 bytes)

Semiconductor memory doubles in size for each address line added to an integrated circuit package, which favors counts that are powers of two. The capacity of a disk drive is the product of the sector size, number of sectors per track, number of tracks per side, and the number of disk platters in the drive. Changes in any of these factors would not usually double the size. Sector sizes were set as powers of two (256 bytes, 512 bytes and so on) for convenience in processing. It was a natural extension to give the capacity of a disk drive in multiples of the sector size, giving a mix of decimal and binary multiples when expressing total disk capacity.

Examples of use

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 English book volume in plain text format (500 pages × 2000 characters per page).

See also

• Binary prefix
• Megabit
• Orders of magnitude (data)
• Timeline of binary prefixes

References

1. ^ ^{a b} The American Heritage Science Dictionary. Houghton Mifflin Company. 2005. http://www.thefreedictionary.com/megabyte. Retrieved 2009-12-22. "1. A unit of computer memory or data storage capacity equal to 1,048,576 bytes (1,024 kilobytes or 2²⁰) bytes. 2. One million bytes. ... prefix mega- often does not have its standard scientific meaning of 1,000,000 ... rate of one megabit per second is equal to one million bits per second ..." 
2. ^ "What are bits, bytes, and other units of measure for digital information? - Knowledge Base". Indiana University. "1MB is 1,024 kilobytes, or 1,048,576 (1024x1024) bytes, not one million bytes. ... Many hard drive manufacturers use a decimal number system to define amounts of storage space. As a result, 1MB is defined as one million bytes, 1GB is defined as one billion bytes, and so on." 
3. ^ ^{a b c d} "Definitions of the SI units: The binary prefixes". National Institute of Standards and Technology. http://physics.nist.gov/cuu/Units/binary.html. Retrieved 2009-12-22. "third megabyte of 1 024 000 bytes is the megabyte used to format the familiar 90 mm (3½ inch), "1.44 MB" diskette" 
4. ^ SanDisk USB Flash Drive "Note: 1 megabyte (MB) = 1 million bytes; 1 gigabyte (GB) = 1 billion bytes."
5. ^ "How Mac OS X reports drive capacity". Apple Inc. 2009-08-27. http://support.apple.com/kb/TS2419. Retrieved 2009-10-16. 

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