US and UK measures are listed separately rather than merged, because a US gallon and a UK gallon differ by 20% and merging them is how recipes and fuel figures go wrong.
US and UK units share names but not sizes. A UK pint is 568 ml; a US pint is 473 ml — a fifth larger, which is why the same recipe behaves differently either side of the Atlantic.
The US gallon is 3.785 litres. The UK imperial gallon is 4.546 litres — about 20% larger. The two diverged in 1824 when Britain redefined its gallon while the United States kept the older wine gallon, and they have never been reconciled.
1 US gal = 3.785411784 L | 1 UK gal = 4.54609 L | 1 UK gal = 1.20095 US gal
This is not academic. Fuel economy figures quoted in miles per gallon are roughly 20% better in UK gallons than US gallons for the identical car, which makes cross-border comparisons meaningless unless you check which gallon is meant.
Fluid ounces disagree too — in the opposite direction
Confusingly, a UK fluid ounce (28.41 ml) is slightly smaller than a US fluid ounce (29.57 ml), even though the UK gallon is bigger. The reason is that the imperial gallon contains 160 fluid ounces while the US gallon contains 128.
For cooking specifically, cup sizes add a third layer of confusion — a US cup is 236.6 ml, a metric cup 250 ml, and an Australian tablespoon is 20 ml against 15 ml elsewhere. If you are converting a recipe by weight rather than volume, the cooking converter handles ingredient densities as well.
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Frequently asked questions
How many litres are in a gallon?
It depends which gallon. A US gallon is 3.785 litres. A UK imperial gallon is 4.546 litres, about 20% larger. Always check which one a figure refers to, particularly with fuel economy.
Is a UK pint bigger than a US pint?
Yes. A UK pint is 568 ml and a US pint is 473 ml, so the UK pint is about 20% larger. A UK pint contains 20 imperial fluid ounces; a US pint contains 16 US fluid ounces.
How many millilitres are in a US cup?
A US cup is 236.588 ml, usually rounded to 237 ml or 240 ml on nutrition labels. A metric cup, used in Australia and New Zealand, is exactly 250 ml.
Why is a UK fluid ounce smaller than a US one when the UK gallon is bigger?
Because the two systems divide the gallon differently. The imperial gallon splits into 160 fluid ounces, while the US gallon splits into 128. The UK ounce is 28.41 ml against the US 29.57 ml.
What is an oil barrel in litres?
A standard oil barrel is 42 US gallons, which is 158.99 litres. It is used for crude oil pricing and is different from other barrel measures used for beer or dry goods.