Length units: overview and conversion factors
Metric system, imperial units and special measurements
The International System of Units (SI) defines the metre as the base unit of length. Since 1983, a metre has been defined via the speed of light: the distance light travels in a vacuum in 1/299,792,458 of a second. All metric units derive from powers of ten: 1 km = 1,000 m, 1 m = 100 cm = 1,000 mm. Before this exact definition, the metre was based on a platinum-iridium prototype bar kept in Sèvres near Paris. The redefinition via the speed of light makes the metre an unchanging physical constant.
The imperial system (US, UK) uses inches, feet, yards and miles. These units have historical roots in body measurements: the foot goes back to the average human foot length, and the inch originally meant "thumb width" and was based on the English king's thumb. Through international agreements, these units were fixed exactly to metric values: 1 inch = 2.54 cm (exact), 1 foot = 12 inches = 30.48 cm, 1 yard = 3 feet = 91.44 cm, 1 mile = 1,760 yards = 1,609.344 m. In everyday life you encounter imperial measurements with US products, screen diagonals and body height.
In maritime and aviation contexts, the nautical mile is the standard. It equals 1,852 m and derives from a geographic arcminute of the Earth's meridian. Speeds at sea are given in knots (kn) – 1 knot = 1 nautical mile per hour = 1.852 km/h. This uniform definition enables worldwide navigation without conversion issues on nautical charts and flight plans.
A common use case is height conversion: in the US and UK, height is given in feet and inches. 5 feet 10 inches (5'10") equals 5 × 30.48 + 10 × 2.54 = 152.4 + 25.4 = 177.8 cm. For screen and TV diagonals, the inch is the international standard measurement: a 65-inch TV has a diagonal of 65 × 2.54 = 165.1 cm. In construction, screws, pipes and tools are often sized in inches, even in Europe – a relic of the British origins of industrialization.
When converting, watch out for common sources of error: the statute mile (1,609.344 m, road traffic) and the nautical mile (1,852 m, seafaring) are different units. Likewise, the rod (roughly 3.7–4.7 m, historically variable by region) should not be confused with the yard (0.9144 m). Our converter works with the internationally valid, exactly defined values and provides results with sufficient precision for technical and everyday applications.