Liters to Gallons: US vs UK — What's the Difference?

Why Two Gallons Exist

Both gallons trace back to English wine and ale measures from the medieval period. The British standardized the "imperial gallon" in 1824 as part of their Weights and Measures Act, defining it as the volume of exactly 10 pounds of water at a specific temperature. This gave 4.54609 liters.

The American gallon was inherited from earlier English wine measures — before the British standardized their imperial system. The US never adopted the imperial gallon after independence, so the two systems diverged. The US gallon of 231 cubic inches (3.785 liters) was codified in US law in the 19th century.

Both gallons are still in active use. The US gallon is used in the United States for fuel, beverages, and liquid products. The imperial gallon is used in the United Kingdom and Ireland for fuel pricing (price per liter is now dominant in retail, but MPG figures still use the imperial gallon) and in a few other Commonwealth countries.

Conversion Factors

1 US gallon = 3.78541 liters. 1 imperial (UK) gallon = 4.54609 liters. To go the other way: 1 liter = 0.264172 US gallons = 0.219969 imperial gallons.

For mental math: 1 US gallon ≈ 3.8 liters. 1 UK gallon ≈ 4.5 liters.

Fuel Economy: The Gallon Problem

Fuel economy figures are not directly comparable between the US and UK because they use different gallons. A UK car rated at 40 MPG (imperial) uses 7.1 liters per 100 km. A US car rated at 40 MPG (US gallon) uses 5.9 liters per 100 km. The UK car is more fuel efficient, even though the MPG number is the same.

Europe and most of the world use liters per 100 km (L/100km) for fuel economy — which avoids the gallon ambiguity entirely. When comparing international fuel efficiency, always convert to L/100km for an apples-to-apples comparison.

Canada uses liters and kilometers, so Canadian fuel economy uses L/100km. Australia uses the same. Only the US and UK still publish official fuel economy in MPG — and they use different gallons.

Everyday Volume Examples

US beverage sizes: a half-gallon of milk = 1.89 liters. A gallon of juice = 3.785 liters. Gas station fuel is sold by the US gallon — when US prices are quoted per gallon, remember this is 3.785 liters.

In the UK, beer was traditionally sold in pints. 1 imperial pint = 568 mL (vs. 473 mL for a US pint). This means a UK pint of beer is about 20% larger than a US pint — another divergence from the same gallon split.

Which gallon does Canada use? Neither. Canada uses liters for fuel and beverages. Despite its historical British connection, Canada metricated in the 1970s–80s and switched to liters. A Canadian gas station sells fuel in cents-per-liter.

Conclusion

The US gallon (3.785 L) and the UK imperial gallon (4.546 L) differ by about 20% — enough to make fuel economy figures incomparable without knowing which gallon is being used. For international comparisons, liters or L/100km are unambiguous. When a recipe, fuel spec, or product measurement cites "gallons," always check which country it comes from.