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The prefix kilo is derived from the Greek word χίλιοι (chilioi), 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.
- one kilogram is 1000 grams
- one kilometre is 1000 metres
- one kilojoule is 1000 joules
- one kilobaud is 1000 bauds
- one kilohertz is 1000 hertz
- one kilobit is 1000 bits
- one kilobyte (kB) is 1000 bytes (see exception below for KB)
A second definition has been in common use in some fields of computer science and information technology, which is, however, inconsistent with the SI. It uses kilo as meaning 210 = 1024, because of the mathematical coincidence that 210 is approximately 103. 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 prefixes have been introduced which are based on powers of 2. In that system, 1024 bytes are called a kibibyte or 1 KiB.
When units occur in exponentiation, such as in square and cubic forms, any multiplier prefix is considered part of the unit, and thus included in the exponentiation.
- 1 km2 means one square kilometre or the area of a square that measures 1000 m on each side or 106 m2 (as opposed to 1000 square meters, which is the area of a square that measures 31.6 m on each side).
- 1 km3 means one cubic kilometre or the volume of a cube that measures 1000 m on each side or 109 m3 (as opposed to 1000 cubic meters, which is the volume of a cube that measures 10 m on each side).