A US gallon is 3.785 litres. An Imperial gallon is 4.546 litres. That is a 20% difference hiding behind one word, and it is responsible for more bad conversions than any other unit ambiguity in everyday use.
It also does not stop at the gallon. Every subdivision below it differs too — quarts, pints and even the fluid ounce — and not by the same proportion, because the two systems chop the gallon into different numbers of ounces.
The whole family, side by side
| Unit | US customary | Imperial (UK) | Difference |
|---|---|---|---|
| Gallon | 3.785 L | 4.546 L | UK is 20% larger |
| Quart | 0.946 L | 1.137 L | UK is 20% larger |
| Pint | 473 mL | 568 mL | UK is 20% larger |
| Fluid ounce | 29.57 mL | 28.41 mL | UK is 4% smaller |
| Fluid ounces per pint | 16 | 20 | — |
Look at the fluid ounce row: it reverses. The Imperial fluid ounce is the smaller of the two, even though every larger Imperial measure is bigger. That is because the Imperial pint is divided into 20 fluid ounces while the US pint is divided into 16. Anyone who assumes "UK measures are bigger" and applies it to fluid ounces gets it backwards.
Why there are two
England historically used several gallons at once — one for wine, one for ale, one for corn. The American colonies inherited the wine gallon of 231 cubic inches, and the United States kept it after independence. Britain swept the whole set away in 1824 and defined a single Imperial gallon as the volume of ten pounds of water at a specified temperature, which landed at about 4.546 litres.
So the split is not the US diverging from a standard. Both countries kept a different one of England's several gallons. Today both are defined exactly: the US gallon as 231 cubic inches (and hence exactly 3.785411784 L, since the inch is exactly 2.54 cm), and the Imperial gallon as exactly 4.54609 L since 1985.
There is also a third: the US dry gallon, about 4.405 L, used for produce and grain. It is rare, but it turns up in agricultural statistics — and it is closer in size to the Imperial gallon than to the US liquid gallon, which makes it an especially good trap.
Where the 20% actually bites
Fuel economy figures
This is the biggest one. A car rated 40 MPG in the UK is not the same as a car rated 40 MPG in the US — the UK figure is measured against a gallon that is 20% larger, so it flatters the car. A US 35 MPG converts to about 42 UK MPG for the identical vehicle. If you are comparing cars across the Atlantic, convert both to L/100km first, which is what the fuel economy guide covers in detail.
Recipes
A British recipe calling for a pint of milk means 568 mL. An American one means 473 mL. Following a UK recipe with US measures leaves you nearly 100 mL short on every pint — enough to change a batter noticeably.
Beer
A UK pint is 568 mL and a US pint is 473 mL. The difference is about a fifth of a glass, and it is the most frequently noticed instance of this whole problem.
Tank and container capacity
Aquariums, fuel cans, water butts and drums are all sold by the gallon in both markets. A "55-gallon drum" in US terms is 208 L; the same phrase read Imperially would be 250 L. Dosing anything by volume — water treatment, fertiliser, fuel additive — inherits the error directly.
How to tell which gallon a figure means
- Check the country of origin first. US sources mean the US liquid gallon unless they say otherwise. UK sources mean the Imperial gallon.
- Look for the litre figure. Most modern documents give both. If a gallon is stated as roughly 3.8 L it is US; roughly 4.5 L is Imperial.
- Count the ounces. If a pint is 16 fl oz it is US; if it is 20 fl oz it is Imperial. This works even when nobody mentions gallons.
- Watch for "Imp" or "UK". Fuel economy figures in particular are often labelled "mpg (Imp)" precisely because the ambiguity is well known in that field.
- Note the date. Canada, Australia and New Zealand all used Imperial gallons before metrication. Older documents from those countries are Imperial; modern ones are metric.
The practical advice
When you have a choice, work in litres. It is the only volume unit in this conversation that means one thing everywhere, it composes cleanly with mass for water, and it removes the ambiguity entirely rather than managing it. Convert to gallons only at the point where you have to speak to a gallon- shaped audience — and say which gallon you mean when you do.