Why a US Gallon Is Not a UK Gallon (And What Else Differs)
The US and UK gallons differ by about 20%. The pint, the fluid ounce and the ton diverge too — and the fluid ounce diverges in the opposite direction.
Of all the traps in unit conversion, shared names with different values is the one that catches careful people. A US gallon and a UK gallon are both "a gallon". They differ by about 20%.
The numbers
- US liquid gallon: 3.785411784 litres (exactly)
- UK (imperial) gallon: 4.54609 litres (exactly)
The imperial gallon is about 20.1% larger. Fill a "10 gallon" tank with the wrong definition and you are out by nearly eight litres.
How the split happened
Pre-1824 England had a mess of gallons: a wine gallon of 231 cubic inches, an ale gallon of 282, and a corn gallon of about 268. Different trades used different vessels and the law tolerated all of them.
The newly independent United States inherited this system and standardised on the Queen Anne wine gallon of 231 cubic inches, which was the one used for most commercial liquids.
Britain reformed separately. The Weights and Measures Act 1824 swept away the old gallons and defined a single imperial gallon as the volume of ten pounds of distilled water at 62 °F — about 277.42 cubic inches. Elegant in principle, and completely different from the American figure.
The fluid ounce goes the other way
Here is the part that surprises people. The gallon is bigger in the UK, but the fluid ounce is bigger in the US:
- US fluid ounce: 29.5735 mL
- UK fluid ounce: 28.4131 mL
The reason is the subdivision. A US gallon contains 128 fluid ounces; a UK gallon contains 160. The UK divides a larger gallon into more, smaller parts.
So the US fluid ounce is about 4% larger, while the US gallon is about 17% smaller. The direction of the error reverses depending on which unit you are converting — which is precisely why guessing is dangerous.
The pint
A US liquid pint is 473 mL; a UK pint is 568 mL. The imperial pint is about 20% larger, and a British pub pint really does deliver noticeably more beer than an American one.
The subdivisions are the same in both systems — 8 pints to the gallon, 2 pints to the quart — so the whole ladder scales together.
The dry gallon, a third system
The US also maintains a separate dry measure for grain and produce, based on the Winchester bushel. The US dry gallon is about 4.405 litres — different from both liquid gallons.
A US dry pint is roughly 551 mL against a liquid pint's 473 mL. When an American recipe calls for "a pint of blueberries", it means the dry pint, which is a volume of container rather than a volume of liquid. We keep these in a separate dry volume converter to avoid mixing them.
Tons: three units, one word
Mass has the same problem:
- Short ton (US) = 2,000 lb = 907.18 kg
- Long ton (UK) = 2,240 lb = 1,016.05 kg
- Tonne (metric) = 1,000 kg = 2,204.62 lb
The spread between short ton and long ton is 12%. In shipping, freight pricing and commodity contracts, that is a material sum, which is why serious contracts always specify which ton they mean.
Practical rules
- Check the source's nationality. A recipe from an American site uses US cups and ounces; a British one uses imperial.
- Prefer metric when precision matters. A millilitre is a millilitre everywhere. Serious baking is done by weight in grams for exactly this reason.
- Watch fuel economy. UK miles-per-gallon figures are inherently higher than US ones for the same car, because the gallon is bigger. Our fuel consumption converter keeps them separate.
- Specify in writing. "Gallons" in a contract is ambiguous. "US gallons" is not.