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.

Look up kilo- in Wiktionary, the free dictionary.

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.^[1][2] As an opponent of suggestions to introduce the metric system in Britain, he qualified the nomenclature adopted in France as barbarous.

• one kilogram (kg) is 1000 grams
• one kilometre (km) is 1000 metres
• one kilojoule (kJ) is 1000 joules
• one kilolitre (kL) is 1000 litres
• one kilobaud (kBd) is 1000 bauds
• one kilohertz (kHz) is 1000 hertz
• one kilodalton (kDa) is 1000 daltons
• one kilobit (kb) is 1000 bits
• one kilobyte (kB) is 1000 bytes
• one kiloohm is (kΩ) is 1000 ohms
• one kilosecond (ks) is 1000 seconds
• one kilotonne (kt) is 1000 tonnes

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

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

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.

• 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