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The Collaborative International Dictionary

Kilo- \Kil"o-\pref. [F. kilo-. See Kilogram.] A combining form used to signify thousand in forming the names of units of measurement; as, kilogram, kilometer, kilowatt, etc.

Douglas Harper's Etymology Dictionary

word-forming element meaning "one thousand," introduced in French 1795, when the metric system was officially adopted there, from Greek khilioi "thousand," of unknown origin.


pre. 1 In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 1,000. Symbol: k 2 (context computing English) Multiplying the unit to which it is attached by 210 (1024)


Kilo (from the Greek , literally a thousand) is a decimal unit prefix in the metric system denoting multiplication by one thousand (10). It has been used in the International System of Units where it has the unit symbol k, in lower case.

The prefix kilo is derived from the Greek word , meaning "thousand". It was originally adopted by Antoine Lavoisier's research group in 1795, and introduced into the metric system in France with its establishment in 1799. Based on a proposal by Thomas Young, the prefix was often alternatively spelled chilio- in early 19th century sources.


A second definition has been in common use in some fields of computer science and information technology, which is, however, inconsistent with the SI definition. It uses kilo as meaning 2 = 1024, because of the mathematical coincidence that 2 is approximately 10. The reason for this application is that binary values natively used in computing are base 2 and not the base 10 which is used for the SI prefixes. The NIST comments on this confusion: "Faced with this reality, the IEEE Standards Board decided that IEEE standards will use the conventional, internationally adopted, definitions of the SI prefixes", instead of kilo for 1024.


  • One " kilobyte" (kB) is 1024 bytes in JEDEC-standard, whereas the definition has shifted to, in most contexts, mean 1000 bytes (kB) in accordance with SI.

To address this confusion, a new set of binary prefixes have been introduced which are based on powers of 2. In that system, 1024 bytes are called a kibibyte or 1 KiB.