An imperial (UK) gallon is 4.54609 liters and a US gallon is 3.785411784 liters, both exactly. That makes the imperial gallon about 20% bigger: 1 imperial gallon = 1.201 US gallons. The difference carries through to pints and quarts, and it’s why British cars report higher mpg figures than the same cars sold in the US.
Why are the two gallons different? #
Both descend from old English gallons, of which there used to be several. The US kept the English wine gallon of 231 cubic inches, and since the inch is exactly 2.54 cm, that works out to exactly 3.785411784 L (NIST SP 811).
Britain replaced its assortment of gallons in 1824 with a single imperial gallon, originally defined as the volume of 10 pounds of water under set conditions. Today it’s fixed at exactly 4.54609 L by the Weights and Measures Act 1985. The US had already gone its own way, so it never adopted the new gallon.
US vs imperial: gallons, quarts, pints and fluid ounces #
Both systems split the gallon into 4 quarts and 8 pints. They differ in the fluid ounce: the US pint has 16 US fluid ounces, while the imperial pint has 20 imperial fluid ounces.
| Unit | US | Imperial (UK) | UK ÷ US |
|---|---|---|---|
| Gallon | 3.785 L | 4.546 L | 1.201 |
| Quart | 946 mL | 1,137 mL | 1.201 |
| Pint | 473 mL | 568 mL | 1.201 |
| Fluid ounce | 29.57 mL | 28.41 mL | 0.961 |
| Fluid ounces per pint | 16 | 20 | |
| Fluid ounces per gallon | 128 | 160 |
So the UK fluid ounce is actually about 4% smaller than the US one, but there are more of them in each pint, and the UK pint ends up 20% bigger. An imperial gallon holds 153.7 US fluid ounces, and a US gallon holds 133.2 imperial fluid ounces.
Converting between them #
- Imperial gallons to US gallons: multiply by 1.20095.
- US gallons to imperial gallons: multiply by 0.83267.
- Either to liters: multiply by 4.54609 (imperial) or 3.785411784 (US).
Gallon conversion charts #
| US gallons | Liters | Imperial gallons |
|---|---|---|
| 1 | 3.79 | 0.83 |
| 5 | 18.93 | 4.16 |
| 10 | 37.85 | 8.33 |
| 15 | 56.78 | 12.49 |
| 20 | 75.71 | 16.65 |
| 50 | 189.27 | 41.63 |
| Imperial gallons | Liters | US gallons |
|---|---|---|
| 1 | 4.55 | 1.20 |
| 5 | 22.73 | 6.00 |
| 10 | 45.46 | 12.01 |
| 15 | 68.19 | 18.01 |
| 20 | 90.92 | 24.02 |
| 50 | 227.30 | 60.05 |
Where does the difference show up? #
Fuel economy. A car goes farther on a bigger gallon, so UK mpg figures are about 20% higher than US mpg for the same car. 40 mpg in a British review is 33.3 US mpg. To compare properly, convert both to L/100km, as shown in mpg to L/100km.
Beer. A British pint is 568 mL. An American pint is 473 mL. Order “a pint” in London and you get about 95 mL more than in New York.
Milk and recipes. A British recipe that calls for “1 pint of milk” means 568 mL, which is 2.4 US cups, not 2. See US cup vs UK cup for the cup side of this.
Aquariums and ponds. Older British fishkeeping books size tanks in imperial gallons. A 20 US gallon tank is 16.65 imperial gallons, so a dosing instruction written per UK gallon needs adjusting.
Fuel prices. Most countries, the UK and Canada included, sell fuel by the liter today. To compare a price per liter with a US price per gallon, multiply by 3.785: 1.50 per liter is 5.68 per US gallon. Currency is a separate step.
Worked examples #
A British car magazine says 50 mpg. 50 × 0.83267 = 41.6 US mpg.
A US recipe for 2 gallons of brine, made in a UK kitchen. 2 US gallons = 7.57 L, which is 1.67 imperial gallons.
A 5-gallon water jug in the US. 18.9 L, or 4.16 imperial gallons.
Converting gallons on your phone #
Convert Units Calculator lists US and UK gallons, pints and fluid ounces as separate units in version 1.0.5, next to liters and milliliters, so you see both answers at once instead of guessing which gallon a source meant. The same version’s Fuel category converts US mpg, UK mpg, L/100km and km/L. It uses the exact definitions above and works offline.
For plain liter conversions, see liters to gallons, and for small volumes, ml to oz.
Frequently asked questions #
Is a UK gallon bigger than a US gallon? #
Yes. An imperial gallon is 4.546 L and a US gallon is 3.785 L, so the UK gallon is about 20% bigger.
How many liters are in an imperial gallon? #
Exactly 4.54609 liters.
Why is UK mpg higher than US mpg? #
Because the imperial gallon holds about 20% more fuel, the same car travels about 20% more miles per gallon. Nothing about the car changes, only the size of the gallon.
Is a UK pint bigger than a US pint? #
Yes. A UK pint is 568 mL (20 imperial fluid ounces), and a US pint is 473 mL (16 US fluid ounces).
Does Canada use US or imperial gallons? #
Canada officially uses metric and sells fuel by the liter. When older Canadian sources mention gallons, they usually mean imperial gallons, reflecting Canada’s British measurement history. Check the source if it matters.