Unit Conversion Calculators
7 free unit conversion calculators, each explaining its formula.
Exact conversions between metric and imperial units, with the definitions and conversion factors the numbers come from.
Length Converter
Since 1959 the inch has been defined as exactly 25.4 millimetres, which makes every imperial length conversion exact rather than a…
Unit ConversionWeight Converter
Includes stones and pounds, which almost no international converter handles properly.…
Unit ConversionTemperature Converter
The only common conversion where you cannot simply multiply, because two of the scales do not start from the same zero.…
Unit ConversionSpeed Converter
Includes knots and Mach, plus the pace equivalents runners and cyclists actually use.…
Unit ConversionData Storage Converter
There are two competing definitions of a gigabyte, and the gap between them is exactly why a '1 TB' drive shows up as 931 GB.…
Unit ConversionVolume Converter
A US pint is 473 ml and a UK pint is 568 ml. Recipes and fuel figures both go wrong when that is missed.…
Unit ConversionArea Converter
Includes acres and hectares, the two units that dominate land measurement and confuse everyone who does not use them daily.…
About this section
Unit conversion is the one area where the numbers are exact. Since the international agreements of 1959, an inch is precisely 25.4 millimetres and a pound is precisely 0.45359237 kilograms — these are definitions rather than measurements, so the conversions involve no approximation at all.
The difficulty lies entirely in ambiguity of naming. There are two gallons in common use and they differ by twenty per cent. There are two fluid ounces and the larger one belongs to the system with the smaller gallon. There are at least four cups. There are three different tons. A megabyte means one of two things depending on whether a storage manufacturer or an operating system is speaking, and the gap compounds to ten per cent at terabyte scale.
These are not pedantic distinctions. They are the reason an American recipe halved with a British measuring jug fails, the reason a fuel economy figure looks twenty per cent better in one country than another for an identical car, and the reason a drive sold as one terabyte reports as 931 gigabytes.
Each converter below lists the exact factor it uses, explains where the unit came from, and flags the places where two units share a name and not a value.