Volume is the measure where British and American units differ the most. Both countries use pints, gallons and fluid ounces, but the sizes aren't the same. The UK imperial gallon was defined in 1824 and is now exactly 4.54609 litres. The US gallon descends from an older English wine gallon and is about 3.785 litres. Because the smaller units are fractions of these, almost every UK and US volume measure has a different metric value.
Recipes and cups
American recipes measure most ingredients by volume in cups. A US cup is about 237 millilitres. Australian and some British recipes use a "metric cup" of 250 millilitres. That 5% difference rarely matters for liquids, but for baking it can. Flour is the main problem: a cup of flour can weigh anywhere from about 120 to 150 grams depending on how it's scooped. For baking, converting American cup measures to grams using the ingredient's typical weight gives more reliable results than converting to millilitres.
Spoons vary too. A UK tablespoon is 15 millilitres and a teaspoon 5 millilitres, which also matches the US measures closely. An Australian tablespoon, though, is 20 millilitres, so check where a recipe comes from.
Fluid ounces and pints
A UK fluid ounce is about 28.4 millilitres, and a US fluid ounce about 29.6 millilitres. Pints differ more, because a UK pint is 20 UK fluid ounces while a US pint is 16 US fluid ounces. A UK pint is about 568 millilitres; a US pint about 473.
Worked conversions
An American recipe asking for 2 cups of milk needs about 473 millilitres. A 750 ml bottle of wine is about 1.32 UK pints. A 330 ml can is about 0.58 of a UK pint. And 50 litres of fuel is about 11 UK gallons, or about 13.2 US gallons, which matters when comparing fuel economy figures from American sources.
Medicines and small volumes
Small volumes need the most care. Liquid medicines in the UK are dosed in millilitres, and pharmacies supply oral syringes or 5 ml spoons for accuracy. Household teaspoons vary widely in size, so the NHS advises against using them for medicine. If an overseas leaflet gives a dose in fluid ounces or "teaspoons", convert to millilitres and measure with a syringe.
Checking your answer
A UK pint is a little over half a litre, about 568 millilitres. A litre is a little under two UK pints, about 1.76. A UK gallon is about four and a half litres, and a US gallon about three and three-quarters. A cubic metre is a thousand litres, roughly the volume of water in five or six full bathtubs. If a converted kitchen measure comes out in cubic metres, or a swimming pool in millilitres, check the prefix and the direction of the conversion.
Volumes on food labels
UK labels must show volume for liquids and weight for solids in metric units, and many also show imperial figures. Milk is the familiar exception that still comes in pints, often labelled with both: a four-pint bottle is 2.272 litres.
One more worked example
A paddling pool 2 metres across, filled to a depth of 30 centimetres, holds about π × 1² × 0.3, or roughly 0.94 cubic metres. That's about 940 litres, around 207 UK gallons, and weighs nearly a tonne. At a garden tap delivering perhaps 10 to 15 litres a minute, it takes over an hour to fill.
Engine sizes
Car engine capacity is a volume too, usually given in litres or cubic centimetres. A 1.6-litre engine has a displacement of 1,600 cc. Older American and classic British engines are sometimes described in cubic inches: a 350 cubic inch engine is about 5.7 litres.