An imperial gallon is 4.546 litres. A US gallon is 3.785 litres. Same name, same country of origin, and one is a fifth bigger than the other. 1 imperial gallon = 1.201 US gallons. 1 US gallon = 0.833 imperial gallons.
Both trace back to England, which is exactly why the split is confusing. Before 1824, England used several different "gallon" definitions depending on what was being measured: wine, ale, and grain each had their own. The American colonies took the wine gallon (231 cubic inches) with them, and the newly independent United States kept using it after 1776.
In 1824, Britain reformed its measurements and defined a single new imperial gallon as the volume of 10 pounds of water at a specified temperature, a bigger, cleaner unit that replaced all the old English gallons at once. The US never adopted the change, so it's still using the pre-1824 wine gallon today, more than two centuries after the split.
| Unit | US customary | Imperial (UK) |
|---|---|---|
| Gallon | 3.785 L | 4.546 L |
| Quart | 946.4 mL | 1,136.5 mL |
| Pint | 473.2 mL, 16 fl oz | 568.3 mL, 20 fl oz |
| Fluid ounce | 29.5735 mL | 28.4131 mL |
| Fluid ounces per gallon | 128 | 160 |
A US dry gallon also exists, at 4.405 litres. It is used for grain and produce in the United States and never for liquids, so it is not part of the comparison above.
Look at the last two rows again. The imperial gallon is the bigger of the two, but the imperial fluid ounce is the smaller of the two. That inversion is what catches people out.
It happens because the two systems cut the gallon into a different number of pieces. An imperial gallon is 160 imperial fluid ounces. A US gallon is 128 US fluid ounces. So the imperial gallon is about 20% larger while its fluid ounce is about 4% smaller.
The practical consequence: "8 fl oz" in an American recipe is 236.6 mL, and the same phrase in an old British or Australian one is 227.3 mL. The pint is the bigger trap, because it is 16 fl oz in the US and 20 in imperial, which is why a UK pint of beer is 568 mL against an American 473 mL.
UK and US miles per gallon are not the same number, because they are not the same gallon. A UK figure is always the larger one for identical fuel use.
To convert, multiply UK mpg by 0.833. Worked through: 40 UK mpg is 33.3 US mpg. Going the other way, multiply US mpg by 1.201, so 30 US mpg is 36.0 UK mpg. A car advertised at 50 mpg in Britain is doing about 41.6 mpg by American measurement, and neither figure is wrong.
For buying fuel, no. Petrol and diesel have been sold in litres since 1995. For describing fuel economy, yes: mpg remains the figure quoted in British car reviews, manufacturer specifications and everyday conversation, even though nobody buys a gallon of anything. The result is a country that measures consumption in a unit it no longer sells.
Canada made the same switch and mostly dropped the gallon with it, quoting consumption as litres per 100 km instead. That is a third convention again, and it runs in the opposite direction: a lower number is better.