|SI decimal prefixes||Binary
|IEC binary prefixes|
|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|
The megabyte (symbol MB, sometimes abbreviated as Mbyte) is a multiple of the unit byte for digital information storage or transmission with three different values depending on context: 1048576 bytes (220) generally for computer memory; one million bytes (106, see prefix mega-) generally for computer storage; and in rare cases 1000×1024 (1024000) bytes. The IEEE Standards Board has confirmed that mega- means 1000000, with exceptions allowed for the base-two meaning if defined explicitly.
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 to the SI prefix mega-, it began to be used for binary multiples as well. In 1998 the International Electrotechnical Commission (IEC) proposed standards for binary prefixes requiring the use of megabyte to strictly denote 10002 bytes and mebibyte to denote 10242 bytes. By the end of 2009, the IEC Standard had been adopted by the IEEE, EU, ISO and NIST. Nevertheless, the term megabyte continues to be widely used with different meanings:
- Base 10
- 1 MB = 1000000 bytes (= 10002 B = 106 B) 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. 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.
- Base 2
- 1 MB = 1048576 bytes (= 10242 B = 220 B) is the definition used by Microsoft Windows in reference to computer memory (e.g. RAM). See Consumer confusion (in the "gigabyte" article). This definition is synonymous with the unambiguous IEC standard name mebibyte.
- 1 MB = 1024000 bytes (= 1000×1024) B is the definition 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.
Semiconductor memory doubles in size for each address lane 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 (most common 512 bytes or 4096 bytes) 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 
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).
- a 4 megapixel JPEG image with normal compression is about 1 MB in size.
- About 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 
- The American Heritage Science Dictionary. Houghton Mifflin Company. 2005. 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 ..."
- "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."
- "Definitions of the SI units: The binary prefixes". National Institute of Standards and Technology. 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"
- SanDisk USB Flash Drive "Note: 1 megabyte (MB) = 1 million bytes; 1 gigabyte (GB) = 1 billion bytes."
- "How Mac OS X reports drive capacity". Apple Inc. 2009-08-27. Retrieved 2009-10-16.
- Christley, S. .; Lu, Y. .; Li, C. .; Xie, X. . (2008). "Human genomes as email attachments". Bioinformatics 25 (2): 274–275. doi:10.1093/bioinformatics/btn582. PMID 18996942.
- 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