Quick answer: An imperial (UK) gallon is 4.54609 litres, while a US liquid gallon is 3.785411784 litres. That makes the imperial gallon about 20% bigger, a difference of roughly 0.76 litres. One imperial gallon equals 1.201 US gallons, and one US gallon equals 0.833 imperial gallons. Both contain 8 pints, but the pints differ.
Two countries use the same word for two different volumes, and the gap shows up in fuel economy figures, American recipes and product labels. This explainer covers where each gallon came from, how every smaller unit differs, and how to convert between them without mistakes. For the wider family of volume units, start at our volume and capacity conversions hub.
How big is an imperial gallon compared with a US gallon?
An imperial gallon is 1.20095 US gallons, so it holds just over a fifth more. Going the other way, a US gallon is 0.83267 of an imperial gallon, about five-sixths. The two exact definitions are:
- Imperial gallon: 4.54609 litres exactly, set by the Weights and Measures Act 1985 (about 277.42 cubic inches).
- US liquid gallon: 231 cubic inches exactly. Since the inch is 25.4 mm, that works out at 3.785411784 litres.
Subtract one from the other and the gap is 0.7607 litres, a little more than a 750 ml wine bottle. Whenever a source simply says “gallon”, it is worth checking which country it was written for before you trust the number. The UK figure is explained in more detail in our guide to the UK gallon in litres.
Why are the UK and US gallons different?
Because Britain reformed its gallons in 1824 and the United States kept an older English one. Before the reform, England used several gallons at once. The wine gallon was fixed in 1707 at 231 cubic inches, while the ale gallon stayed at 282 cubic inches, and grain was measured with yet another standard.
The American colonies inherited this mix. After independence, the United States settled on the 231-cubic-inch wine gallon for liquids, plus a separate dry gallon for grain. Britain went a different way: the Weights and Measures Act 1824 swept the old gallons away and introduced a single imperial gallon, defined as the volume of ten pounds of water under specified conditions. That new gallon fell between the old wine and ale gallons, so the two systems have differed ever since.
Today both are defined through the metre. The imperial gallon is a fixed number of litres, and the US gallon is a fixed number of cubic inches, each inch being exactly 25.4 mm. Neither will drift, but neither will they ever match.
Do the pints, quarts and fluid ounces differ too?
Yes: every unit derived from the gallon keeps the same 20% gap, except the fluid ounce, which flips the other way. Both systems split the gallon into 4 quarts and 8 pints, so a UK pint is 20% bigger than a US pint. But the UK divides its gallon into 160 fluid ounces and the US divides its smaller gallon into 128, which leaves the US fluid ounce about 4% larger.
| Unit | UK imperial | US customary (liquid) | UK ÷ US |
|---|---|---|---|
| Gallon | 4,546.09 ml | 3,785.41 ml | 1.201 |
| Quart (¼ gallon) | 1,136.52 ml | 946.35 ml | 1.201 |
| Pint (⅛ gallon) | 568.26 ml | 473.18 ml | 1.201 |
| Gill (¼ pint) | 142.07 ml | 118.29 ml | 1.201 |
| Fluid ounce | 28.41 ml (1/160 gal) | 29.57 ml (1/128 gal) | 0.961 |
| Fluid ounces per pint | 20 | 16 | — |
For the smaller units, see how many ml are in a UK pint, or use the volume converter and choose UK or US units from the list.
How do you convert imperial gallons to US gallons?
Multiply imperial gallons by 1.20095 to get US gallons, or multiply US gallons by 0.83267 to get imperial gallons. If you only need a rough answer, “add a fifth” turns imperial into US, and “take off a sixth” turns US into imperial.
| Imperial gallons | US gallons | Litres |
|---|---|---|
| 1 | 1.201 | 4.546 |
| 5 | 6.005 | 22.730 |
| 10 | 12.009 | 45.461 |
| 20 | 24.019 | 90.922 |
| 50 | 60.047 | 227.305 |
| 100 | 120.095 | 454.609 |
How does the difference affect miles per gallon?
A car’s UK mpg figure is about 20% higher than its US mpg figure, even though the car uses exactly the same fuel. A bigger gallon simply takes you further. Multiply US mpg by 1.201 to get UK mpg, or divide UK mpg by 1.201 to get US mpg.
| US mpg | UK mpg |
|---|---|
| 20 | 24.0 |
| 25 | 30.0 |
| 30 | 36.0 |
| 35 | 42.0 |
| 40 | 48.0 |
| 50 | 60.0 |
This matters when you read an American road test of a car that is also sold in Britain: the economy figure will look poor until you convert it. Litres per 100 km avoids the problem entirely, because litres and kilometres are the same everywhere.
Where might a UK reader meet a US gallon?
- US fuel prices. Petrol in America is priced per US gallon. To compare with a UK pump price per litre, divide the US price by 3.785.
- American recipes and drinks. Batch recipes for punch, brine or soap often use US gallons and US quarts.
- Imported products. Paint, pool chemicals and garden sprayers made for the US market may state capacity in US gallons; the litre figure on the label is the reliable one.
- Online forums. Fishkeeping, home-brewing and motoring forums mix British and American members, so a “50 gallon tank” might mean 227 litres or 189 litres.
Common mistakes to avoid
- Assuming “gallon” means the same thing everywhere. Check the source country, or look for a litre figure.
- Scaling fluid ounces by 1.2. The pint and gallon ratio does not apply to ounces; a UK fluid ounce is slightly smaller than a US one.
- Confusing the US dry gallon with the liquid gallon. The US dry gallon (about 4.405 litres) is used for grain and produce, not for drinks or fuel.
- Comparing mpg figures across the Atlantic without converting. A 30 mpg American figure is 36 mpg in UK terms.
Frequently asked questions
Which is bigger, the imperial gallon or the US gallon?
The imperial gallon is bigger. It holds 4.546 litres compared with 3.785 litres for a US liquid gallon, about 20% more.
How many US gallons are in an imperial gallon?
One imperial gallon equals 1.20095 US gallons. Multiply imperial gallons by 1.20095 to convert to US gallons.
Does Canada use the imperial gallon or the US gallon?
Canada sells fuel and most liquids in litres. Older Canadian references to gallons generally mean the imperial gallon, but always check for a litre figure.
Is a UK pint bigger than a US pint?
Yes. A UK pint is 568 ml and a US pint is 473 ml, the same 20% gap as between the two gallons.
Why is US mpg lower than UK mpg for the same car?
Because the US gallon is smaller, the car travels fewer miles on it. Multiply US mpg by 1.201 to get the equivalent UK mpg.
Sources
- Weights and Measures Act 1985, Schedule 1, legislation.gov.uk
- NIST Guide to the SI, Appendix B.9 (UK and US gallon, quart, pint and fluid ounce factors)
- Encyclopaedia Britannica: wine gallon (1707 definition and the ale gallon)
- National Physical Laboratory, the UK’s national measurement institute
More Volume and Capacity guides
- How Many ml in a Tablespoon UK? 15 ml and Chart
- How Many ml in a Teaspoon UK? 5 ml Explained
- Litres to Pints UK: Converter and Printable Chart
- UK Fluid Ounce vs US Fluid Ounce: What’s the Difference?
- Imperial Gallon vs US Gallon: The Difference Explained
- UK Gallon in Litres: Converter, Chart and mpg Guide