Common Converters

How these converters work

Each converter turns your value into a shared base unit, then out again into the unit you asked for. Length works through the metre, mass through the kilogram, and so on. Doing it in two steps is what lets any unit in a category convert to any other without a separate factor for every possible pair.

Enter a number, choose the unit you are converting from and the unit you want, and the result appears immediately. Every page also shows the factor applied and a table of the most common values, so you can check the arithmetic yourself or use the table without a calculator.

Metric, imperial and US customary

Most of the world measures in metric units, where each step up or down is a factor of ten: a kilometre is 1,000 metres, a centimetre is one hundredth of a metre. The imperial and US customary systems grew out of older English measures and use irregular steps instead, such as 12 inches to a foot and 5,280 feet to a mile.

The two systems overlap in awkward ways. A US gallon is about 3.785 litres while an imperial gallon is about 4.546 litres, and a US fluid ounce differs slightly from an imperial one. Where a unit has more than one definition, the converter states which it uses, so a recipe, a fuel figure or a shipping weight ends up in the units you actually need.

Accuracy and rounding

Some conversions are exact by definition. One inch is exactly 2.54 centimetres, and one pound is exactly 0.45359237 kilograms, because those values were fixed by agreement. Others, like converting between degrees and radians, involve an irrational number and can only ever be approximated.

Results are shown to enough decimal places for everyday and most professional use. If your work has a required tolerance, round at the end rather than partway through: rounding an intermediate value and then converting again is the most common way small errors creep into a calculation.

Temperature is the exception

Almost every conversion here is a simple multiplication, but temperature is not. Celsius, Fahrenheit and Kelvin have different zero points, so converting between them needs an offset as well as a scale: Fahrenheit to Celsius subtracts 32 before multiplying by 5/9. This is why doubling a temperature in one scale does not double it in another, and why temperature differences convert differently from temperature readings.