US Gallons vs Imperial Gallons: Why a UK Gallon Is 20% Bigger
A US gallon is 3.785 litres and an imperial gallon is 4.546 litres. That 20% gap changes fuel economy, beer and recipe figures.
7 September 2026 · 5 min read · Reference
The United States and the United Kingdom share the word "gallon" but not the size. The difference is large enough to matter at the fuel pump, in the pub and in the kitchen.
Exact sizes
| Unit | US | Imperial (UK) |
|---|---|---|
| Gallon | 3.785411784 L | 4.54609 L |
| Quart | 946.35 mL | 1,136.52 mL |
| Pint | 473.18 mL | 568.26 mL |
| Cup | 236.59 mL | 284.13 mL |
| Fluid ounce | 29.57 mL | 28.41 mL |
1 imperial gallon = 1.20095 US gallons
Why they differ
Before 1824, Britain used several gallons, including a wine gallon and an ale gallon. The United States standardised on the old 231 cubic inch wine gallon. Britain replaced all of them with the new imperial gallon, originally defined as the volume of 10 pounds of water.
There is a twist in the smaller units. An imperial gallon holds 160 imperial fluid ounces while a US gallon holds 128 US fluid ounces. That makes the imperial fluid ounce slightly smaller than the US one, even though every larger imperial unit is bigger.
Fuel economy
UK car figures in miles per gallon use imperial gallons, and US figures use US gallons. The same car looks about 20% more efficient in the UK:
| US mpg | UK mpg | L/100 km |
|---|---|---|
| 25 | 30.0 | 9.4 |
| 30 | 36.0 | 7.8 |
| 40 | 48.0 | 5.9 |
| 50 | 60.0 | 4.7 |
Always check which gallon a figure uses before comparing vehicles.
The pint
A UK pub pint is 568 mL. A US pint glass holds 473 mL. Order "a pint" in each country and the British one is about 95 mL bigger.
Related conversions
This article is for general information. Please verify figures against the relevant standard before using them in critical work. About accuracy