Length measures distance between two points. Convert between meters, feet, inches, kilometers, miles, and more.
Length conversions cover the basic units of distance: metres, feet, inches, miles, and kilometres, along with the exact ratios that connect the metric and imperial systems. Anyone comparing a European height or room dimension to a US measurement, or checking a foreign altitude or nautical distance, needs to know which units are exact conversions and which are older, discontinued definitions.
The metre has not been tied to a physical object since 1983. It is defined as the distance light travels in a vacuum in 1/299,792,458 of a second, a definition anchored to the speed of light rather than to a bar of metal kept in a vault. Earlier definitions used a platinum-iridium bar held in France, but that bar could change size by fractions of a micrometre over decades, which is not acceptable for a unit meant to be exact everywhere on Earth.
Because the metre is now defined through a physical constant, it does not degrade or drift the way a manufactured artifact can. Any lab with a sufficiently precise clock and a vacuum can reproduce the exact same metre without ever consulting the original French bar, which was retired once the light-based definition took over.
Before 1959, the inch was defined slightly differently in the US, the UK, and other English-speaking countries, with gaps of a few parts per million between them. The international yard and pound agreement of that year fixed the inch at exactly 2.54 centimetres across the US, UK, Canada, Australia, South Africa, and New Zealand, replacing several near-identical national inches with a single agreed value. The foot, at 12 inches, became exactly 0.3048 metres as a direct consequence.
The older US survey foot differed from the new international foot by about two parts per million, a gap too small to matter for almost anything except land surveying over long distances. The US formally retired the survey foot at the end of 2022, after which US land surveys use the same international foot as everyone else.
Metric units cover most of the world's day to day measurement, but two fields kept imperial units by international convention rather than habit. Aviation altitude is reported in feet almost everywhere, including in countries that use metric units for everything else, a standard set by early international civil aviation bodies that stuck regardless of a country's usual system.
At sea, distance is measured in nautical miles, each exactly 1,852 metres, a unit tied to one minute of latitude rather than to the metre or the mile used on land. A nautical mile is about 15 percent longer than the statute mile used for road distances, and confusing the two produces a meaningfully wrong distance on a chart.