Kilo-

For other uses, see Kilo (disambiguation).

Kilo (from the Greek χίλιοι, literally a thousand) is a decimal unit prefix in the metric system denoting multiplication by one thousand (10³). It has been 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] Error: {{Lang}}: text has italic markup (help)), 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

Examples

• one kilogram is 1000 grams
• one kilometre is 1000 metres
• one kilojoule is 1000 joules
• one kilobaud is 1000 bauds
• one kilohertz is 1000 hertz
• one kilobit is 1000 bits
• one kilobyte (kB) is 1000 bytes

kilobyte

For the kilobyte, a second definition has been in common use in some fields of computer science and information technology, which is, however, inconsistent with the SI definition. It uses kilobyte as meaning 2¹⁰ bytes = 1024 bytes, because of the mathematical coincidence that 2¹⁰ is approximately 10³. The reason for this application is that binary values natively used in computing are base 2 values, and not decimal based. The NIST comments on this confusion: "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.

Example:

• One "kilobyte" (kB) is 1024 bytes in JEDEC-standard, whereas the definition has shifted to, in most contexts, mean 1000 bytes (kB) in accordance with SI.

To address this confusion, a new set of binary prefixes have been introduced which are based on powers of 2. In that system, 1024 bytes are called a kibibyte or 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)

References

SI prefixes

Prefix		Base 10	Decimal	Adoption
Name	Symbol
quetta	Q	1030	1000000000000000000000000000000	2022
ronna	R	1027	1000000000000000000000000000
yotta	Y	1024	1000000000000000000000000	1991
zetta	Z	1021	1000000000000000000000
exa	E	1018	1000000000000000000	1975
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
quecto	q	10−30	0.000000000000000000000000000001
Notes