Unit converter
Nine kinds of unit, metric and US or imperial, with the working shown
Pick what you are converting, type an amount and choose the units. The example turns 5 kilometers into miles.
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The same amount in every unit
How the converter works
Every unit is stored as an exact multiple of one base unit for its kind: meters for length, kilograms for weight, liters for volume, square meters for area, meters per second for speed, bytes for data, joules for energy and watts for power. To convert, the amount is multiplied into the base unit and divided out into the unit you want. The working line under the answer shows those steps with the real numbers, so you can repeat them on any calculator.
Where the numbers come from
The factors are the legal definitions, not rounded look-ups: 1 inch is exactly 2.54 cm and 1 pound exactly 0.45359237 kg (the 1959 international agreement, as listed by the US National Institute of Standards and Technology in Handbook 44 and Special Publication 811). Metric units follow the SI Brochure of the International Bureau of Weights and Measures. The imperial gallon is 4.54609 liters under UK law.
Units that differ by country
Gallons, pints, fluid ounces, cups, tablespoons, tons and horsepower come in more than one size. Each size is listed on its own, such as US gallon and imperial gallon, and a note appears whenever you pick one. Temperature uses formulas rather than a single factor; see the temperature converter.
Rounding
The answer is worked out from the exact definition, then shown to 4 decimal places (6 significant figures for numbers below 1). It says "exactly" only when nothing was rounded.