Kilometer

Kilometer (symbol: km) is a unit of length in the metric system, equal to 1000 metres. It is a derived SI unit formed by the prefix kilo- (10^3) applied to the metre. By definition, 1 kilometre = 1000 metres. The kilometre is widely used for measuring long distances on Earth, such as road distances, travel routes, and geographic extents. It is convenient for maps and road signs. The SI unit for speed in everyday use is kilometre per hour (km/h), though metres per second (m/s) is also common in technical contexts.

Conversions and context: 1 km ≈ 0.621371 miles; 1 mile ≈ 1.60934 km. A variety of everyday distances—such as a few kilometres between towns or a multi-kilometre cycling route—illustrate its practical use. In sports, racing distances are often described in kilometres (e.g., 5 km, 10 km). Distances in astronomy and space science are typically given in larger units, while kilometres remain convenient for local, terrestrial measurements.

Etymology and classification: the word kilometre comes from Greek chilioi ("thousand") and metron ("measure"). The kilometre is not an SI base unit; metres is the base unit of length, and the kilometre is a widely used derived unit within the SI framework.

History: the kilometre arose with the adoption of the metric system in the late 18th and early 19th centuries. The metre was defined as one ten-millionth of the distance from the equator to the North Pole, and the kilometre became a standard practical unit for longer distances.