Kilo-

For other uses, see Kilo.

Kilo is a unit prefix in the metric system of measurement, denoting multiplication by one thousand (10³). The International System of Units reserves the lowercase symbol k.

Kilo is derived from the Greek word χίλιοι (chilioi), meaning "thousand".

In 19th century English the prefix 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.

Examples

• 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 sometimes also preceded by the prefix:

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

kilobyte

For multiples of the byte in some fields of computer science and information technology, another definition has been in common use, in which the kilobyte measures 1024 bytes (2¹⁰ bytes), because 2¹⁰ is approximately 10³. The reason for this application is that digital hardware natively use base 2 exponentiation.

The National Institute of Standards and Technology (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." ^[3] A new set of binary prefixes, based on powers of 2, was introduced by the International Electrotechnical Commission (IEC), which defines 1024 bytes as one kibibyte (1 KiB).

Exponentiation

When units occur in exponentiation, such as in square and cubic forms, any multiplier prefix is 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

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

References

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

SI prefixes

• v
• t
• e

Prefix		Base 10	Decimal	Adoption [nb 1]
Name	Symbol
quetta	Q	1030	1000000000000000000000000000000	2022 [1]
ronna	R	1027	1000000000000000000000000000
yotta	Y	1024	1000000000000000000000000	1991
zetta	Z	1021	1000000000000000000000
exa	E	1018	1000000000000000000	1975 [2]
peta	P	1015	1000000000000000
tera	T	1012	1000000000000	1960
giga	G	109	1000000000
mega	M	106	1000000	1873
kilo	k	103	1000	1795
hecto	h	102	100
deca	da	101	10
—	—	100	1	—
deci	d	10−1	0.1	1795
centi	c	10−2	0.01
milli	m	10−3	0.001
micro	μ	10−6	0.000001	1873
nano	n	10−9	0.000000001	1960
pico	p	10−12	0.000000000001
femto	f	10−15	0.000000000000001	1964
atto	a	10−18	0.000000000000000001
zepto	z	10−21	0.000000000000000000001	1991
yocto	y	10−24	0.000000000000000000000001
ronto	r	10−27	0.000000000000000000000000001	2022 [1]
quecto	q	10−30	0.000000000000000000000000000001
Notes Prefixes adopted before 1960 already existed before SI. The introduction of the centimetre–gram–second system of units was in 1873.

1. On the extension of the range of SI prefixes. CGPM. Resolution 3 of the 27th CGPM (2022). BIPM. 18 November 2022. doi:10.59161/cgpm2022res3e . Retrieved 5 February 2023.
2. "Metric (SI) Prefixes". NIST.