|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 kilobyte (symbol: kB) is a multiple of the unit byte for digital information. Although the prefix kilo- means 1000, the term kilobyte and symbol kB have historically been used to refer to either 1024 (210) bytes or 1000 (103) bytes, dependent upon context, in the fields of computer science and information technology.
The term "kilobyte" is commonly used to mean either 10001 bytes or 10241 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. In 1998 the International Electrotechnical Commission (IEC) proposed standards for binary prefixes and requiring the use of megabyte to strictly denote 10002 bytes and mebibyte to denote 10242 bytes. By the end of 2007, the IEC Standard had been adopted by the IEEE, EU, and NIST. Nevertheless, the term kilobyte continues to be widely used with the following two different meanings:
Base 10 definition 
- 1 kB = 1000 bytes (= 10001 B = 103 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 definition 
- 1 KB = 1024 bytes (= 10241 B = 210 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 kibibyte.
- The HP 21MX real-time computer (1974) denoted 196,608 (which is 192×1024) as "196K", while the HP 3000 business computer (1973) denoted 131,072 (which is 128×1024) as "128K".
- The Shugart SA-400 51⁄4-inch floppy disk (1976) held 109,375 bytes unformatted, and was advertised as "110 Kbyte", using the 1000 convention. Likewise, the 8-inch DEC RX01 floppy (1975) held 256,256 bytes formatted, and was advertised as "256k". On the other hand, the Tandon 51⁄4-inch DD floppy format (1978) held 368,640 bytes, but was advertised as "360 KB", following the 1024 convention.
- On modern systems, Mac OS X Snow Leopard represents a 65,536 byte file as "66 KB", rounding to the nearest 1000, while Microsoft Windows 7 would divide by 1024 and represent this as "64 KB".
In December 1998, the IEC addressed such multiple usages and definitions by creating unique binary prefixes to denote multiples of 1024, such as “kibibyte (KiB)”, which represents 210, or 1024, bytes. However, adoption by the computer industry has not been universal, particularly when communicating in a public context, such as to consumers of computer or computer based products.
Examples of use 
- 1 kilobyte: very short story
- 2 kilobytes: typewritten page
- 10 kilobytes: page from an encyclopedia
See also 
- definition of kilobyte from Oxford Dictionaries Online. Askoxford.com. Retrieved on 2011-01-07.
- Kilobyte – Definition and More from the Free Merriam-Webster Dictionary. Merriam-webster.com (2010-08-13). Retrieved on 2011-01-07.
- Kilobyte | Define Kilobyte at Dictionary.com. Dictionary.reference.com (1995-09-29). Retrieved on 2011-01-07.
- Prefixes for Binary Multiples — The NIST Reference on Constants, Units, and Uncertainty
- 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.
- Frankenberg, Robert (October 1974). "All Semiconductor Memory Selected for New Minicomputer Series" (PDF). Hewlett-Packard Journal (Hewlett-Packard) 26 (2): pg 15–20. Retrieved 2007-06-18. "196K-word memory size"
- Hewlett-Packard (November 1973). "HP 3000 Configuration Guide" (PDF). HP 3000 Computer System and Subsystem Data: pg 59. Retrieved 2010-01-22.
- National Institute of Standards and Technology. "Prefixes for binary multiples". "In December 1998 the International Electrotechnical Commission (IEC) [...] approved as an IEC International Standard names and symbols for prefixes for binary multiples for use in the fields of data processing and data transmission."
- Mueller, Scott. Upgrading and Repairing PCs. p. 596. ISBN 0-7897-2974-1.
- Raymer, Michael G. The Silicon Web: Physics for the Internet Age. p. 40. ISBN 978-1-4398-0311-0.
- Knuth: Recent News. Cs-staff.stanford.edu. Retrieved on 2011-01-07.
- Atwood, Jeff. (2007-09-10) Gigabyte: Decimal vs. Binary. Coding Horror. Retrieved on 2011-01-07.
- Huggins, James. "How Much Data Is That?". jamesshuggins. Retrieved 27 September 2011.