A US gallon is 3.785 litres. Both conversion factors are worth holding: multiply gallons by 3.785 to get litres, and multiply litres by 0.264 to go back.
The catch is that "gallon" names two different volumes. The imperial gallon used in the UK is 4.54609 litres — 20% larger than the US gallon — so a conversion is only right once you know which gallon you were handed.
| Gallons | US gallons in litres | Imperial gallons in litres |
|---|---|---|
| 0.5 | 1.893 L | 2.273 L |
| 1 | 3.785 L | 4.546 L |
| 2 | 7.571 L | 9.092 L |
| 3 | 11.356 L | 13.638 L |
| 5 | 18.927 L | 22.730 L |
| 10 | 37.854 L | 45.461 L |
| 15 | 56.781 L | 68.191 L |
| 20 | 75.708 L | 90.922 L |
| Litres | US gallons | Imperial gallons |
|---|---|---|
| 1 | 0.264 | 0.220 |
| 2 | 0.528 | 0.440 |
| 5 | 1.321 | 1.100 |
| 10 | 2.642 | 2.200 |
| 20 | 5.283 | 4.399 |
| 40 | 10.567 | 8.799 |
| 50 | 13.209 | 10.998 |
| 60 | 15.850 | 13.198 |
Handy at a European pump: a 50-litre fill is about 13.2 US gallons, and a typical 60-litre tank is just under 16 US gallons.
Three different gallons
The word covers three legally distinct volumes:
- US liquid gallon: exactly 3.785411784 L (231 cubic inches). Fuel, milk, paint — everything sold by the gallon in the United States
- Imperial gallon: exactly 4.54609 L. Still used in the UK for fuel economy, and in a handful of Caribbean and other Commonwealth contexts
- US dry gallon: 4.405 L. A rarely-used grain measure; you will only meet it as one-eighth of a US bushel
The imperial gallon is 20.1% larger than the US one. Both are exact definitions rather than measurements, so conversions between them are exact too: 1 imperial gallon = 1.20095 US gallons.
Why the two gallons differ
They descend from different English measures. Britain historically kept several gallons at once — a wine gallon of 231 cubic inches, a corn gallon, and an ale gallon — and the United States, when it standardised, kept the wine gallon.
Britain went the other way. The Weights and Measures Act of 1824 replaced the lot with a single imperial gallon, defined as the volume of 10 pounds of distilled water at a specified temperature and pressure. That produced a larger unit, and it has been fixed at exactly 4.54609 litres since 1985.
The practical upshot: the two systems have not lined up for two centuries, and every "gallon" figure needs its origin checked before it is converted.
The fuel-economy trap
Because a UK gallon is 20% bigger, UK mpg figures look 20% better than US ones for exactly the same car. A car rated 40 mpg in Britain is 33.3 mpg in the US — the vehicle has not changed, only the gallon.
To convert to litres per 100 km, divide a constant by the mpg figure:
- From US mpg: 235.215 ÷ mpg
- From imperial mpg: 282.481 ÷ mpg
So 30 US mpg is 7.84 L/100 km, and 30 imperial mpg is 9.42 L/100 km. Note that mpg and L/100 km run in opposite directions — higher mpg is better, lower L/100 km is better — so you cannot compare the two figures by eye.
Sizes you meet in practice
Common containers, converted:
- 1 US gallon of milk = 3.785 L
- Half-gallon carton = 1.893 L
- 5-gallon water-cooler bottle = 18.93 L
- 55-gallon drum = 208.2 L
- 1 litre bottle = 0.264 US gallons, so about 3.8 litre bottles fill a gallon
- 2 litre soda bottle = 0.528 US gallons, so a gallon is just under two of them
For mental arithmetic, treating a gallon as "about 3.8 litres" and a litre as "about a quart plus 5%" gets you within a couple of percent — close enough for a fuel tank, not close enough for a chemistry protocol.
Frequently Asked Questions
How many litres are in a gallon?
A US gallon is exactly 3.785411784 litres, or 3.785 L in practice. An imperial (UK) gallon is exactly 4.54609 litres — about 20% larger.
How many gallons is 1 litre?
0.264172 US gallons, or 0.219969 imperial gallons. Roughly, a litre is a quarter of a US gallon plus about 6%.
Why is a UK gallon bigger than a US gallon?
They come from different English measures. The US kept the old wine gallon of 231 cubic inches (3.785 L), while Britain’s 1824 Weights and Measures Act defined a new imperial gallon as the volume of 10 pounds of water (4.546 L). The imperial gallon has been 20.1% larger ever since.
How many litres is 5 gallons?
18.927 litres for 5 US gallons, or 22.730 litres for 5 imperial gallons. A 5-gallon water-cooler bottle is the US figure — just under 19 litres.
How do I convert mpg to litres per 100 km?
Divide 235.215 by the US mpg figure, or 282.481 by an imperial mpg figure. So 30 US mpg is 7.84 L/100 km. Remember the scales invert: higher mpg is better, lower L/100 km is better.
Is a gallon of water the same as a gallon of petrol?
The volume is identical — a gallon is a gallon. The weight is not: a US gallon of water weighs about 8.34 pounds (3.79 kg), while a gallon of gasoline weighs roughly 6.1 pounds (2.8 kg) because it is less dense.