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 symbol k, in lowercase.

The prefix kilo is derived from the Greek word χίλιοι (chilioi), meaning "thousand".

In 19th century English it was sometimes spelled chilio, in line with a puristic opinion by Thomas Young. As an opponent of suggestions to introduce the metric system in Britain, he qualified the nomenclature adopted in France as barbarous.

## Examples

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

• one kiloeuro (k€) is 1000 euros
• one kilodollar (k\$) is 1000 dollars

## kilobyte

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. 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).

## Exponentiation

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).

• milli- (inverse of kilo- prefix, denoting a factor of 1/1000)
• kibi- (binary prefix, denoting a factor of 1024)
• RKM code
1. ^ Brewster, David (1832). The Edinburgh Encyclopaedia. Vol. 12 (1st American ed.). Joseph and Edward Parker. Retrieved 2015-10-09.
2. ^ Dingler, Johann Gottfried (1823). Polytechnisches Journal (in German). Vol. 11. Stuttgart, Germany: J.W. Gotta'schen Buchhandlung. Retrieved 2015-10-09.
3. ^ Definition of binary prefixes at NIST
[nb 1]
Name Symbol
quetta Q 1030 1000000000000000000000000000000 2022
ronna R 1027 1000000000000000000000000000 2022
yotta Y 1024 1000000000000000000000000 1991
zetta Z 1021 1000000000000000000000 1991
exa E 1018 1000000000000000000 1975
peta P 1015 1000000000000000 1975
tera T 1012 1000000000000 1960
giga G 109 1000000000 1960
mega M 106 1000000 1873
kilo k 103 1000 1795
hecto h 102 100 1795
deca da 101 10 1795
100 1
deci d 10−1 0.1 1795
centi c 10−2 0.01 1795
milli m 10−3 0.001 1795
micro μ 10−6 0.000001 1873
nano n 10−9 0.000000001 1960
pico p 10−12 0.000000000001 1960
femto f 10−15 0.000000000000001 1964
atto a 10−18 0.000000000000000001 1964
zepto z 10−21 0.000000000000000000001 1991
yocto y 10−24 0.000000000000000000000001 1991
ronto r 10−27 0.000000000000000000000000001 2022
quecto q 10−30 0.000000000000000000000000000001 2022
Notes
1. ^ Prefixes adopted before 1960 already existed before SI. The introduction of the CGS system was in 1873.
1. ^ "On the extension of the range of SI prefixes". 18 November 2022. Retrieved 5 February 2023.