Kilometer

A kilometer (abbreviated km) is a unit of length equal to one thousand meters. It is one of the units in the metric system (SI), which established the meter as the basic unit of length. The prefix 'kilo' means thousand, and it functions in the same way for other units such as kilogram.

The kilometer is the most commonly used unit worldwide for measuring distances on roads, distances between cities, and vehicle speed (kilometers per hour, km/h). A few countries, including the United States and the United Kingdom, still use the mile (mile) in many contexts instead of the kilometer.

Relationship to other units

One kilometer equals 1,000 meters, 100,000 centimeters, or 1,000,000 millimeters. One thousand kilometers make up one megameter, a unit rarely used. When converted to the imperial system, one kilometer is approximately 0.621 miles, while one mile is approximately 1.609 kilometers.

Origin and history

The meter was originally defined during the French Revolution in the late eighteenth century, based on a portion of the distance between the North Pole and the equator. According to that definition, Earth's entire circumference is approximately 40,000 kilometers. Today the meter is defined as the distance light travels in an extremely small fraction of a second, making it a precise definition that does not depend on Earth's measurements.

Uses

The kilometer is used on road signs, maps, and travel planning. Sports such as long-distance running and cycling measure distances in kilometers. Similarly, the square kilometer (km²) is a unit of area, commonly used to measure the area of countries and cities. In astronomy, very large distances are measured in larger units such as light-years, because numbers expressed in kilometers would be extremely large.