How to convert a unit
- Enter the value you want to convert.
- Choose the unit you are starting from.
- Choose the unit you want it in.
- Read the result and the factor in both directions.
- Scan the list below for the same quantity in every other unit.
What this converter handles
- Distance in metres, kilometres, feet and miles.
- Volume in litres, millilitres and both kinds of gallon.
- UK pints, which differ from US pints by a fifth.
- The conversion factor shown in both directions.
- Every other unit listed alongside, so you rarely need a second conversion.
- Live results with nothing sent anywhere.
The two gallons problem
A US gallon is 3.785 litres. A UK gallon is 4.546 litres. That is a difference of about 20 per cent, and both are called a gallon.
The consequence turns up constantly. American fuel economy quoted in miles per gallon is not comparable with British figures, 30 mpg in the US is roughly 36 mpg in the UK for the same car. Recipes written in American cups and pints do not scale to British measures. Paint coverage, tank capacities and chemical dilutions all carry the same risk.
The pint follows the gallon, so a UK pint is 568 millilitres against the US 473. That is why a pint of beer in London is noticeably larger than one in New York, and it is not a rounding difference.
Why the systems diverged
Both descend from English units, but the United States kept the older wine gallon of 231 cubic inches after independence, while Britain redefined its gallon in 1824 as the volume of ten pounds of water. The two never reconverged, and the difference has propagated into every derived unit since.
Length units did reconverge. The international agreement of 1959 fixed the inch at exactly 25.4 millimetres for both, which is why distance conversions are unambiguous and volume conversions are not.
Units that only look interchangeable
- Fluid ounces: the US and imperial versions differ by about four per cent.
- Cups: 240 ml in the US, 250 ml in Australia, and not a standard unit in the UK at all.
- Tons: metric, US short and UK long tons all differ.
- Miles: statute and nautical miles differ by 15 per cent.
The safe habit with any recipe or specification from another country is to check which system the units belong to before converting, rather than after the result looks wrong.
Converting is not the same as scaling
Conversion changes the unit while keeping the quantity identical: two litres and 2,000 millilitres are the same amount of liquid. Scaling changes the quantity while keeping proportions, which is what doubling a recipe does.
Confusing the two produces some memorable results. If a recipe needs scaling as well as converting, do the conversion first and the scaling second, the ratio calculator handles the proportional part, and doing it in that order keeps the arithmetic checkable at each step.
Precision and rounding
Conversion factors here are exact where an exact definition exists and rounded only in the display. That matters when converting twice: going metric to imperial and back should return your original number, and it does, because the underlying arithmetic never uses the rounded figure you see.
For dedicated length and mass conversions with the full range of units, the length converter and weight converter cover more ground. Everything runs in your browser.