A US gallon is 3.785 liters. An imperial gallon is 4.546 liters. Same word, same four letters on every pump nozzle in the English-speaking world, and a 20% difference you'll only notice when a number doesn't match what you expected.
How we ended up with two gallons
In 1707, Britain had something called the "wine gallon" — 231 cubic inches, about 3.785 liters. The American colonies inherited it and have kept using it to this day. Britain itself reformed its measurement system in 1824, creating the imperial gallon: 4.546 liters, based on 10 pounds of water at a specific temperature. The US never adopted the new one. Canada did. Australia did. Nobody bothered to rename anything, so both systems use the word "gallon".
The cascading effect
The gallon difference flows through every US vs imperial volume:
- Gallon — US 3.785 L, imperial 4.546 L.
- Quart — US 0.946 L, imperial 1.136 L.
- Pint — US 0.473 L, imperial 0.568 L.
- Fluid ounce — US 29.57 mL, imperial 28.41 mL (this one flips!).
Why does the fluid ounce flip? Because a US gallon has 128 fl oz, while an imperial gallon has 160. The US made the gallon smaller but used fewer, bigger ounces. The UK made the gallon bigger but used more, smaller ounces.
Where it matters
Fuel economy is the big one. A car that gets 40 MPG in the US gets reported as ~48 MPG in the UK for identical real-world mileage — British MPG looks artificially good because the denominator is 20% bigger. When comparing cars across countries, convert to L/100km first.
Recipes are the other common trap. A British recipe calling for "1 pint of milk" means 568 mL. A US recipe for the same dish means 473 mL. That's 95 mL of difference — enough to change a custard's texture.
US gal ↔ imperial gal ↔ liters, plus every other volume. Shareable URL for the receipt.