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For other uses, see Kilo (disambiguation).

Kilo (from the Greek χίλιοι, literally a thousand) is a decimal unit prefix in the metric system denoting multiplication by one thousand (103). 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 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.

In 19th century English it was sometimes spelled chilio, in line with a puristic opinion by Thomas Young[1][2]


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 210 = 1024, because of the mathematical coincidence that 210 is approximately 103. 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.[3]


  • 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.


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).

See also[edit]

  • milli- (inverse of kilo- prefix, denoting a factor of 1/1000)
  • kibi- (binary prefix, denoting a factor of 1024)


  1. ^ Brewster, David (1832). The Edinburgh Encyclopaedia. 12 (1st American ed.). Joseph and Edward Parker. Retrieved 2015-10-09. 
  2. ^ Dingler, Johann Gottfried (1823). Polytechnisches Journal (in German). 11. Stuttgart, Germany: J.W. Gotta'schen Buchhandlung. Retrieved 2015-10-09. 
  3. ^ Definition of binary prefixes at NIST
Prefix 1000m 10n Decimal English word Adoption[nb 1]
Name Symbol Short scale Long scale
yotta Y  10008  1024 1000000000000000000000000  septillion  quadrillion 1991
zetta Z  10007  1021 1000000000000000000000  sextillion  thousand trillion or trilliard 1991
exa E  10006  1018 1000000000000000000  quintillion  trillion 1975
peta P  10005  1015 1000000000000000  quadrillion  thousand billion or billiard 1975
tera T  10004  1012 1000000000000  trillion  billion 1960
giga G  10003  109 1000000000  billion  thousand million or milliard 1960
mega M  10002  106 1000000             million 1960 (1873)
kilo k  10001  103 1000             thousand 1960 (1795)
hecto h  10002/3  102 100             hundred 1960 (1795)
deca da  10001/3  101 10             ten 1960 (1795)
 10000  100 1             one
deci d  1000−1/3  10−1 0.1             tenth 1960 (1795)
centi c  1000−2/3   10−2 0.01             hundredth 1960 (1795)
milli m  1000−1  10−3 0.001             thousandth 1960 (1795)
micro μ  1000−2  10−6 0.000001             millionth 1960 (1873)
nano n  1000−3  10−9 0.000000001  billionth  thousand millionth 1960
pico p  1000−4  10−12 0.000000000001  trillionth  billionth 1960
femto f  1000−5  10−15 0.000000000000001  quadrillionth  thousand billionth 1964
atto a  1000−6  10−18 0.000000000000000001  quintillionth  trillionth 1964
zepto z  1000−7  10−21 0.000000000000000000001  sextillionth  thousand trillionth 1991
yocto y  1000−8  10−24  0.000000000000000000000001  septillionth  quadrillionth  1991
  1. ^ The metric system was introduced in 1795 with several metric prefixes, of which, however, only six were adopted as SI prefixes by the 11th CGPM conference in 1960, whereas myria (104) as well as double and demi were not adopted. In 1873, micro and mega were recommended by the British Association for the Advancement of Science. The other dates relate to recognition by a resolution of the CGPM.