Kilometer (km)
Definition
A kilometer (symbol km) is a metric unit of length equal to exactly 1,000 meters. It is one of the most widely used units for measuring road distances and geographic separations in countries that follow the metric system.
History
The kilometer entered use shortly after the meter was defined in 1799, formed by combining the SI prefix kilo (Greek χίλιοι, thousand) with the meter. France formally adopted the metric system on 7 April 1795 (the law of 18 Germinal an III), and the kilometer spread through Europe and Latin America during the 19th century.
Today the kilometer is the standard road-distance unit in almost every country except the United States, the United Kingdom for road signs, and a handful of others. The 2019 SI revision did not change its magnitude; it remains exactly 1,000 meters as derived from the speed-of-light definition of the meter.
Standard reference
Defined by the BIPM SI Brochure (9th edition) as 1,000 × the meter, which is fixed by the speed of light in vacuum (299,792,458 m/s).
Common conversions
| 1 km | = 1,000 m |
|---|---|
| = 0.621371 mi | |
| = 3,280.84 ft | |
| = 100,000 cm |