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Volume Converter
Convert litres, gallons, cups and fluid ounces — and catch the US/imperial mismatch.
An American recipe calls for a gallon of milk. A British reader reaches for a gallon jug and pours in nearly 20% too much, because a US gallon and an imperial gallon are not the same size, despite sharing the exact same name.
Two gallons, two different sizes
A US gallon is 3.785 litres. An imperial (UK) gallon is 4.546 litres — about 20% bigger. Both trace back to old English measures, but the US kept an older definition based on a wine gallon while Britain standardised on a larger one in 1824. Nobody harmonised the two afterward, so both are still in active daily use, just in different countries, both still called simply “gallon.”
The confusion runs deeper than the gallon itself. A US fluid ounce is 29.57 mL. An imperial fluid ounce is 28.41 mL — smaller, even though the imperial gallon is bigger. That is not a contradiction: the US gallon splits into 128 fluid ounces while the imperial gallon splits into 160, so the bigger container is divided into more, smaller pieces.
The numbers that trip people up
| Unit | In litres | Notes |
|---|---|---|
| US gallon | 3.785 L | Fuel, milk and paint in the US |
| Imperial gallon | 4.546 L | Historical UK/Ireland measure, still on some fuel-economy figures |
| US fluid ounce | 29.57 mL | US recipes and drink sizes |
| Imperial fluid ounce | 28.41 mL | Older UK recipes and pub measures |
| US cup | 236.6 mL | US recipes — not a legally defined unit elsewhere |
Where this actually causes problems
- Baking: a US recipe’s “cup” of flour is a volume measure, and even US cooks get inconsistent results because scooping packs flour differently than spooning it in.
- Car fuel economy: older UK figures quoted in miles per imperial gallon look far better than the same car’s US mpg rating, purely because the gallons differ in size, not because the car is more efficient.
- Cocktail and pub measures: a UK pub “shot” and a US bar shot are different volumes, on top of the fluid ounce already being a different size in each country.
- Paint and fuel containers sold as “one gallon” across the US and UK/Ireland genuinely hold different amounts.
Volume conversion questions
Is a US gallon the same as a UK gallon?
No. A US gallon is 3.785 litres and a UK (imperial) gallon is 4.546 litres, about 20% larger. They share a name but not a size, so always check which one a recipe, fuel figure or container is using.
Why is a US fluid ounce different from a UK fluid ounce?
Because each gallon is divided into a different number of ounces: the US gallon splits into 128 fluid ounces, the imperial gallon into 160. That makes the individual imperial fluid ounce smaller (28.41 mL) even though the imperial gallon itself is larger.
How many millilitres are in a US cup?
About 236.6 mL. The cup is a US customary unit and is not a standard measure in the UK or most of the world, where recipes are usually written in grams and millilitres instead.
How do I convert litres to US gallons?
Divide by 3.785. So a 50-litre fuel tank holds about 13.2 US gallons, versus about 11 imperial gallons using the 4.546 figure — the same tank, two different-looking numbers depending on which gallon you use.
Why does the UK still reference gallons at all if it uses litres?
Fuel is sold in litres at the pump, but older fuel-economy figures, especially for classic and imported vehicles, are still sometimes quoted in miles per imperial gallon out of habit and comparability with older data.
