Inches, feet and pounds are the same in the US and the UK. Gallons are not: a US gallon is 3.785 liters, an imperial (British) gallon 4.546 liters. A UK recipe, an old Canadian car manual or a British fuel-economy figure can all be about 20% off if you read them with US units.
Too many gallons
Medieval England had several gallons at once: one for wine, one for ale and beer, and one for dry goods such as grain. Each was defined by a container or a weight of contents, and they differed noticeably.
The US kept the wine gallon
In 1706, under Queen Anne, the English wine gallon was fixed at 231 cubic inches. The American colonies used it, and after independence the United States kept it. Today the US gallon is still defined as exactly 231 cubic inches, which works out to exactly 3.785411784 liters.
Britain started over in 1824
In 1824 the British Weights and Measures Act replaced the jumble with a single imperial gallon: the volume of 10 pounds of water at a stated temperature, about 277.4 cubic inches. Today it is defined as exactly 4.54609 liters. The US was already independent and never adopted the reform.
The fluid ounce runs the other way
Both systems divide the gallon differently, which produces a surprise:
- US: 1 gallon = 4 quarts = 8 pints = 128 fluid ounces, so 1 US fl oz ≈ 29.57 mL.
- Imperial: 1 gallon = 4 quarts = 8 pints = 160 fluid ounces, so 1 imperial fl oz ≈ 28.41 mL.
- So the imperial gallon, quart and pint are about 20% larger, but the imperial fluid ounce is about 4% smaller.
- A US pint is 16 fl oz (473 mL); a British pint of beer is 20 imperial fl oz (568 mL).
Where it matters
- Fuel economy: the same car shows about 20% more “miles per gallon” in UK figures than in US figures.
- Recipes: older British, Canadian and Australian recipes measuring in pints or fluid ounces use imperial sizes.
- Cups: the US customary cup is 236.6 mL, US nutrition labels round it to 240 mL, and many metric recipes use 250 mL.
The volume converter lists US and imperial units separately and switches its vessel to imperial markings whenever you choose an imperial unit, so you can see the difference side by side.
Try it
- Volume converter — cups to gallons, US and imperial