Kilo-

Short description: Decimal unit prefix in the metric system

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

• 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 baud
• one kilohertz (kHz) is 1000 hertz
• one kiloDalton (kDa) is 1000 Dalton
• 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

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).

See also

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

References

SI prefixes

Prefix		Base 10	Decimal	English word		Adoption
Name	Symbol			Short scale	Long scale
yotta	Y	1024	1000000000000000000000000	septillion	quadrillion	1991
zetta	Z	1021	1000000000000000000000	sextillion	trilliard	1991
exa	E	1018	1000000000000000000	quintillion	trillion	1975
peta	P	1015	1000000000000000	quadrillion	billiard	1975
tera	T	1012	1000000000000	trillion	billion	1960
giga	G	109	1000000000	billion	milliard	1960
mega	M	106	1000000	million		1873
kilo	k	103	1000	thousand		1795
hecto	h	102	100	hundred		1795
deca	da	101	10	ten		1795
		100	1	one		–
deci	d	10−1	0.1	tenth		1795
centi	c	10−2	0.01	hundredth		1795
milli	m	10−3	0.001	thousandth		1795
micro	μ	10−6	0.000001	millionth		1873
nano	n	10−9	0.000000001	billionth	milliardth	1960
pico	p	10−12	0.000000000001	trillionth	billionth	1960
femto	f	10−15	0.000000000000001	quadrillionth	billiardth	1964
atto	a	10−18	0.000000000000000001	quintillionth	trillionth	1964
zepto	z	10−21	0.000000000000000000001	sextillionth	trilliardth	1991
yocto	y	10−24	0.000000000000000000000001	septillionth	quadrillionth	1991

he:תחיליות במערכת היחידות הבינלאומית#קילו