Two gallons, one name
An Imperial gallon is about 20.1% larger than a US gallon. They are not variants of the same unit — they are two different measures that happen to share a word, and using the wrong one is the most consequential error in everyday unit conversion.
The divergence is historical. The US gallon descends from the English wine gallon of 231 cubic inches, fixed in 1707 and carried into American law at independence. Britain then abandoned it in 1824 for a new Imperial gallon defined as the volume of ten pounds of water. America kept the older definition, so the split is not America deviating from Britain but Britain changing after America had already copied it.
Where this actually bites
Fuel economy is the worst case. A car rated at 40 miles per gallon in the UK is doing about 33 US mpg for identical real-world performance, purely because the gallon is bigger. Comparing a British review against an American one without adjusting makes the same car look 20% more efficient in Britain. The fuel economy converter handles both systems for exactly this reason.
Recipes are the other common trap, though there the units are pints and fluid ounces rather than gallons. A US pint is 16 US fluid ounces and an Imperial pint is 20 Imperial fluid ounces — so an Imperial pint is roughly 20% more liquid, and a recipe halfway between two conventions produces a genuinely different dish.
Canada uses both, deliberately
Canada sells fuel in litres and has done since metrication, but Imperial gallons persist in older references and in some trades. Anyone converting a Canadian figure from before the 1970s should assume Imperial rather than US, which is the opposite of the instinct most people apply based on geography.
Several Caribbean nations and a few others retain Imperial gallons in specific industries. The safest approach with any gallon figure of unknown origin is to check what a known quantity converts to: if a stated tank capacity works out implausibly large or small, the wrong gallon is the likeliest culprit.
Litres are defined more simply than either
A litre is one cubic decimetre — a cube ten centimetres on each side — and by extension a millilitre is exactly one cubic centimetre. That relationship makes volume, length and mass connect cleanly in metric: one litre of pure water at maximum density weighs almost exactly one kilogram.
No such relationship exists in the imperial system in the US, though the Imperial gallon was originally built on one: ten pounds of water. That definition was abandoned in 1985 in favour of a fixed metric value, so today even the Imperial gallon is defined by reference to the litre.
Frequently asked questions
How many litres in a gallon?
A US gallon is 3.785411784 litres; an Imperial gallon is 4.54609 litres. The Imperial gallon is about 20% larger, and the toggle above selects which one you mean.
Why are US and Imperial gallons different?
The US gallon descends from the English wine gallon of 231 cubic inches, which America kept at independence. Britain replaced it with a new Imperial gallon in 1824, so the split came from Britain changing rather than America deviating.
Does the difference matter for fuel economy?
A great deal. A car rated at 40 mpg in the UK is doing about 33 US mpg for identical performance, purely because the gallon is larger.