UK Gallon vs US Gallon
Why a UK gallon and a US gallon are not the same volume
Understanding the differences between UK (Imperial) Gallon and US Liquid Gallon is essential for choosing the right system for your needs. This comparison covers the key differences, advantages of each approach, and practical guidance on when to use one over the other. Whether you are a student, engineer, scientist, or just curious about measurement systems, this guide will help you understand the trade-offs and make informed decisions.
UK (Imperial) Gallon
Used in the UK and some Commonwealth countries. 1 UK imperial gallon = 4.54609 liters exactly. Defined as the volume of 10 pounds of distilled water at a specific temperature and pressure.
US Liquid Gallon
Used in the United States. 1 US liquid gallon = 3.785411784 liters exactly = 231 cubic inches. The US also has a US dry gallon (≈ 4.405 L), but the liquid gallon is by far the most common.
Key Differences
- 1A UK (imperial) gallon is about 20% larger than a US liquid gallon: 4.546 L vs 3.785 L. 1 UK gallon = 1.20095 US gallons.
- 2Fuel economy figures are directly affected: a car achieving 40 UK mpg achieves only about 33 US mpg, despite no difference in actual efficiency. Always specify which gallon when comparing fuel economy internationally.
- 3Pints and quarts also differ between systems. A US liquid pint is 473 mL; a UK pint is 568 mL. A pint of beer in the UK is therefore about 20% more than a pint in the US.
- 4The US fluid ounce (29.57 mL) and the UK fluid ounce (28.41 mL) are different too, although the difference is smaller.
- 5Gasoline prices in the UK are per liter; in the US, per US gallon. To compare fuel prices directly, multiply UK price per liter by 3.785 to get the equivalent US price per US gallon.
Background
The divergence between UK and US gallons dates to 1824 when the UK standardized the imperial gallon at 10 pounds of water. The US had already standardized its gallon based on the Queen Anne wine gallon (231 cubic inches) established in 1707. The two systems have coexisted independently ever since, creating the persistent confusion around fuel economy comparisons.
When to Use UK (Imperial) Gallon vs US Liquid Gallon
Use UK (Imperial) Gallon when…
Use UK gallons when interpreting UK fuel economy figures, buying fuel in the UK, or following recipes or standards from British sources.
Use US Liquid Gallon when…
Use US gallons for any American context — fuel, milk, paint, and other liquid products sold in the United States are in US gallons.