Type Any Unit Conversion in Plain English

Every unit in each category is stored as a factor against a base unit: the metre for length, kilogram for mass, litre for volume, square metre for area and metre per second for speed. Converting means multiplying by the source factor to reach the base, then dividing by the target factor. Many of the factors used are exact by international definition rather than measured: the inch is exactly 0.0254 m, the foot exactly 0.3048 m, the yard exactly 0.9144 m, the pound exactly 0.45359237 kg, the statute mile exactly 1609.344 m and the nautical mile exactly 1852 m. Temperature is handled separately because the scales are affine rather than proportional. Celsius, Fahrenheit and Kelvin have different zero points, so conversion needs an offset as well as a ratio: C equals F minus 32 times five ninths, and K equals C plus 273.15. Only Kelvin and Rankine have a true zero, so 200 K is meaningfully twice 100 K while 20 degrees C is not twice as hot as 10 degrees C. Other traps: area factors are the square of the corresponding linear factor and volume factors the cube, so a 10 percent linear increase is 21 percent more area and 33 percent more volume. US and imperial liquid measures differ substantially, with the US gallon at 3.785 L against the imperial 4.546 L, and the US pint at 473 mL against 568 mL. A US short ton is 907.18 kg, a metric tonne 1000 kg and a long ton about 1016 kg. This tool uses US customary liquid measures.

The metric system was designed during the French Revolution to replace a patchwork of regional units. A commission of the Academy of Sciences settled on a decimal system tied to nature rather than to a monarch, defining the metre as one ten-millionth of the quarter meridian through Paris, a distance surveyed between Dunkirk and Barcelona by Jean-Baptiste Delambre and Pierre Mechain. The law naming the metre came in 1795, and platinum standards for the metre and kilogram were deposited in 1799. International adoption followed with the Metre Convention of 1875, which created the International Bureau of Weights and Measures, and the modern Systeme International was named at the 11th General Conference on Weights and Measures in 1960. English customary units were later rationalised against those standards. The International Yard and Pound Agreement of 1959, among six Commonwealth and US signatories, fixed the yard at exactly 0.9144 m and the pound at exactly 0.45359237 kg, resolving small national discrepancies; the US survey foot survived in geodesy until it was retired at the end of 2022. The international nautical mile of 1852 m was agreed at a hydrographic conference in Monaco in 1929. The temperature scales are older still, with Daniel Gabriel Fahrenheit's mercury scale of 1724, Anders Celsius's centigrade scale of 1742 and William Thomson's absolute scale of 1848. In 2019 the SI redefined the kilogram, ampere, kelvin and mole in terms of fixed physical constants, ending reliance on physical artefacts.