US gallon vs imperial gallon: why they're different

“Gallon” sounds like it should mean one thing. It doesn’t. A gallon in the United States and a gallon in the United Kingdom are two legally distinct volumes, and the gap between them — about 20%— is large enough to meaningfully skew a fuel-economy comparison, a recipe, or a shipped product’s labelled capacity if you assume they match.

The two definitions

Both figures are exact by legal definition, not approximations — the US gallon’s litre equivalent is set out in NIST Handbook 44, and the imperial gallon’s is fixed in the UK’s Weights and Measures Act 1985. A converter that treats “gallon” as a single unit without asking which one is guaranteed to be wrong for one of the two audiences.

How the split happened

The divergence is historical, not accidental rounding. The gallon the US kept is descended from an old English wine gallon, standardized at exactly 231 cubic inches, which the American colonies used before independence and the newly independent US simply continued using afterward. Britain, meanwhile, overhauled its own measurement system in 1824 and adopted a new, larger imperial gallon, originally defined as the volume occupied by 10 pounds of water at a specified temperature and pressure. From that point forward, the two English-speaking measurement traditions were using differently sized “gallons” under the same name — and neither has since converged with the other.

Where this actually bites: fuel economy

The most common real-world trap is comparing mpg figures across the Atlantic. Because the UK gallon holds about 20% more fuel than the US gallon, the identical car achieving the identical real-world fuel economy will show a highermpg number when quoted in UK gallons than in US gallons — purely from the unit, with zero difference in the car’s actual efficiency. Anyone comparing a US review’s mpg rating against a UK review’s mpg rating for the same vehicle needs to convert both through a common unit — litres per 100 km, or km per litre — rather than compare the raw mpg figures. Our fuel-economy guide covers the EPA-vs-real-world side of that same comparison problem.

The practical rule

When you see “gallon” without a qualifier, assume US gallons if the source is American (fuel pumps, recipes, most retail packaging) and imperial gallons if the source is British or from a country that retained UK measurement conventions. When in doubt, convert through litres — the one volume unit both systems define exactly against, and the unit a liters-to-gallons converter uses as its common reference point.