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Kilo is a decimal unit prefix in the metric system denoting multiplication by one thousand (103). It is 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]


By extension, currencies are also sometimes preceded by the prefix kilo-:


For the kilobyte, a second definition has been in common use in some fields of computer science and information technology. It uses kilobyte to mean 210 bytes (= 1024 bytes), because of the mathematical coincidence that 210 is approximately 103. The reason for this application is that digital hardware and architectures natively use base 2 exponentiation, and not decimal systems. JEDEC memory standards still permit this definition, but acknowledge the correct SI usage.

NIST comments on the confusion caused by these contrasting definitions: "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] To address this conflict, a new set of binary prefixes has been introduced, which is based on powers of 2. Therefore, 1024 bytes are defined as one kibibyte (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.

See also


  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
PrefixBase 1000Base 10 Decimal English wordAdoption [nb 1]
NameSymbol Short scale Long scale
yotta Y 10008  1024 1000000000000000000000000 septillion quadrillion1991
zetta Z 10007  1021 1000000000000000000000 sextillion trilliard1991
exa E 10006  1018 1000000000000000000 quintillion trillion1975
peta P 10005  1015 1000000000000000 quadrillion billiard1975
tera T 10004  1012 1000000000000 trillion billion1960
giga G 10003  109 1000000000 billion milliard1960
mega M 10002  106 1000000 million1873
kilo k 10001  103 1000 thousand1795
hecto h 10002/3  102 100 hundred1795
deca da 10001/3  101 10 ten1795
 10000  100 1 one
deci d 1000−1/3  10−1 0.1 tenth1795
centi c 1000−2/3   10−2 0.01 hundredth1795
milli m 1000−1  10−3 0.001 thousandth1795
micro μ 1000−2  10−6 0.000001 millionth1873
nano n 1000−3  10−9 0.000000001 billionth milliardth1960
pico p 1000−4  10−12 0.000000000001 trillionth billionth1960
femto f 1000−5  10−15 0.000000000000001 quadrillionth billiardth1964
atto a 1000−6  10−18 0.000000000000000001 quintillionth trillionth1964
zepto z 1000−7  10−21 0.000000000000000000001 sextillionth trilliardth1991
yocto y 1000−8  10−24  0.000000000000000000000001 septillionth quadrillionth1991
  1. Prefixes adopted before 1960 already existed before SI. 1873 was the introduction of the CGS system.