Volume Converter

Volume units, and the gallon that is not a gallon

Metric volume is clean: a litre is 1,000 millilitres and also exactly 1,000 cubic centimetres, so litres and cubic measures interchange without friction. Imperial volume is where the difficulty lies, because the United States and the United Kingdom use different gallons. A US liquid gallon is about 3.785 litres; an imperial gallon is about 4.546 litres, roughly twenty per cent larger. The same applies down the scale to pints and fluid ounces.

This is a genuine source of error rather than a curiosity. A recipe calling for a pint of milk means 473 millilitres in American usage and 568 in British. Fuel economy figures quoted in miles per gallon are not comparable between the two systems without conversion, and US figures look worse than they are when read as imperial. Complicating it further, a US fluid ounce is about 29.57 millilitres while an imperial fluid ounce is about 28.41 — so the imperial gallon is larger even though its fluid ounce is smaller, because an imperial gallon contains 160 of them against the US gallon's 128.

Worked example

A car's fuel tank holds 50 litres. In US gallons that is 50 ÷ 3.785 ≈ 13.2 gallons, but in UK (imperial) gallons it is 50 ÷ 4.546 ≈ 11.0 gallons. The same tank sounds 20% bigger in US gallons, simply because the US gallon is smaller.

This also explains why fuel economy figures in miles per gallon look higher in the UK than in the US for the same car.

Common mistakes to avoid

• Assuming a gallon is the same in every country. Check whether a source is American or British.
• Confusing a US pint (473 ml) with a UK pint (568 ml).
• Converting dry ingredients with liquid volume units when a recipe expects weight.

Last reviewed: 5 November 2026 by the YogiCalc team · How we calculate and verify · Spotted an error? Tell us