US vs. Imperial Gallons, Pints, and Ounces: Why the Same Word Means Different Volumes
October 21, 2026
The US and the UK share unit names like pint, quart, and gallon, but the volumes differ. The US customary gallon is defined as 231 cubic inches, which is 3.78541 liters. The British imperial gallon, defined in 1824 as the volume of 10 pounds of water, is 4.54609 liters, about 20.1% larger. The ratio is 1.20095.
The pint is also different: a US pint is 16 US fluid ounces (473.18 mL), while an imperial pint is 20 imperial fluid ounces (568.26 mL). A British pint of beer is therefore about 20% bigger than an American one. And the fluid ounce itself differs slightly, 29.57 mL in the US and 28.41 mL in the imperial system, a ratio of 1.04.
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The pint-quart-gallon relationships also differ in the number of ounces. In the US system, 1 gallon = 4 quarts = 8 pints = 128 fluid ounces. In the imperial system, 1 gallon = 4 quarts = 8 pints = 160 fluid ounces. Because the imperial ounce is slightly smaller but there are 25% more ounces per gallon, the overall result is the 20% difference.
| Unit | US customary | British imperial | Ratio (imperial/US) |
|---|---|---|---|
| Fluid ounce | 29.574 mL | 28.413 mL | 0.961 |
| Pint | 473.176 mL (16 fl oz) | 568.261 mL (20 fl oz) | 1.201 |
| Quart | 946.353 mL | 1,136.52 mL | 1.201 |
| Gallon | 3.78541 L | 4.54609 L | 1.201 |
| Cup | 236.588 mL (8 fl oz) | 284 mL (older British cup, half pint) | 1.20 |
Fuel Economy
Miles per gallon figures in the UK use imperial gallons, which makes them look higher than US figures for the same car. A car that gets 30 mpg in the US gets 30 × 4.54609 ÷ 3.78541 = 36.0 imperial mpg. A UK car rated at 40 mpg corresponds to 40 × 3.78541 ÷ 4.54609 = 33.3 US mpg. Always check which gallon is meant when comparing fuel economy across countries. Most of the world uses litres per 100 km, which avoids the confusion.
Similarly, fuel tank sizes in gallons differ: a 15-gallon US tank holds 56.8 liters, while a 15-gallon imperial tank holds 68.2 liters.
Recipes and Drinks
A British recipe calling for 1 pint of milk means 568 mL. Using a US pint (473 mL) would leave you short by about 17%. A recipe for 1 cup in older British cookbooks might refer to a 10 fluid-ounce imperial cup (284 mL). Modern British recipes mostly use metric units. In Canadian cooking, a cup is 227 mL (based on the imperial gallon) in older recipes and 250 mL today.
The fluid ounce also matters for drinks: an American 12-ounce can holds 355 mL, while a British 12 fl oz would be 341 mL.
Which Countries Use What
The United States uses US customary units. The United Kingdom officially uses metric for most purposes but still uses imperial pints for draught beer and cider and miles for road distances; fuel is sold in litres. Canada officially uses metric, with some imperial usage in daily life. Australia and New Zealand went fully metric in the 1970s. Other countries, such as Liberia and Myanmar, also have historical US-based or imperial usage, though they are gradually moving to metric.
Tips to Avoid Errors
Convert to millilitres or litres when following a recipe from another country. When reading labels, look for the metric equivalent in parentheses. For fuel economy comparisons, convert both to litres per 100 km: 30 US mpg is 7.84 L/100 km, and 36 imperial mpg is also 7.85 L/100 km, the same car.
Frequently Asked Questions
How many litres are in a US gallon and an imperial gallon?
3.785 litres in a US gallon and 4.546 litres in an imperial gallon.
Is a UK pint bigger than a US pint?
Yes, 568 mL versus 473 mL, about 20% larger.
How do I convert imperial mpg to US mpg?
Multiply by 0.8327. For example 40 imperial mpg is 33.3 US mpg.
Why do the two systems differ?
The US kept the older British wine gallon, while Britain redefined its gallon in 1824.
Is a fluid ounce the same in both systems?
No. A US fluid ounce is 29.57 mL, and an imperial fluid ounce is 28.41 mL.