Gallon Converter (US & UK gallons, quarts, pints)

Understanding the gallon converter — concept and principle

The gallon is a volume unit used for fuel, milk, paint and water, but it comes in two different sizes: the US gallon and the UK (imperial) gallon. This converter switches between both gallons, litres, quarts, pints and fluid ounces of each system, so you can read recipes, fuel economy figures and product labels from either country correctly.

1 US gallon = 3.785 L; the UK gallon is 4.546 L, so always check which one is meant.

How it is calculated

Each unit is converted to US gallons and then to the target unit: result = value × (US gal per source unit) ÷ (US gal per target unit). The key definitions are exact:

1 US gallon = 231 cubic inches = 3.785411784 L
1 UK gallon = 4.54609 L, which is about 1.200949926 US gallons
US: 1 gallon = 4 quarts = 8 pints = 128 fluid ounces
UK: 1 gallon = 4 quarts = 8 pints = 160 fluid ounces

The principle

Both gallons go back to old English wine and ale measures, but they were fixed differently. The US kept the 231 cubic-inch wine gallon; because the inch is defined as exactly 2.54 cm, 231 × 2.54³ cm³ = 3,785.411784 cm³. The UK redefined its gallon in 1824 as the volume of 10 pounds of water, and today it is defined directly as 4.54609 L. The UK gallon is therefore about 20% larger. The smaller units differ too: a UK pint holds 20 UK fluid ounces, a US pint 16 US fluid ounces, and the two fluid ounces are not the same size either (about 28.41 mL vs 29.57 mL).

Worked example

Inputs: 1 US gallon, convert to litres.
1 US gallon = 231 in³ × 16.387064 cm³/in³ = 3,785.411784 cm³ = 3.785411784 L.
In UK gallons: 3.785411784 ÷ 4.54609 = 0.8326741846.
US units: 4 quarts, 8 pints, 128 fluid ounces.
UK units: 3.330696739 UK quarts, 6.661393477 UK pints and 133.2278695 UK fluid ounces.

How to read the result

For everyday estimates, a US gallon is about 3.8 L and a UK gallon about 4.5 L. Fuel economy is the classic trap: 30 miles per US gallon is about 36 miles per UK gallon for the same car, because the UK gallon is bigger. When a source does not say which gallon it means, the country of origin is the best clue: US recipes, labels and EPA figures use US gallons; older UK, Irish and some Commonwealth sources use imperial gallons.

Common mistakes

Assuming all gallons are the same; the difference is about 20%.
Converting UK pints with US fluid ounces, or the reverse.
Using the US dry gallon (about 4.405 L), an old unit for grain, for liquids.
Rounding 3.785 L to 4 L in calculations that need accuracy, such as dosing chemicals.

Keep in mind: The factors are exact, but labels often round their own figures. For fuel, medicine or chemical dosing, follow the original unit and the product's instructions rather than a rounded conversion.

Frequently asked questions

How many litres are in a US gallon?

Exactly 3.785411784 L.

How many litres are in a UK gallon?

Exactly 4.54609 L.

Is a UK pint bigger than a US pint?

Yes, about 568.26 mL against 473.18 mL.

Sources: NIST Handbook 44, Appendix C (General Tables of Units of Measurement) · UK Weights and Measures Act 1985, Schedule 1

Checked 8 Nov 2026. NIST Handbook 44 Appendix C; UK Weights and Measures Act 1985, Schedule 1.