The short answer: the UK gallon is 20% bigger
One US liquid gallon = 3.78541 litres. One imperial (UK) gallon = 4.54609 litres. The imperial gallon is 4.54609 ÷ 3.78541 ≈ 1.201 times the US gallon — so a UK gallon holds a fifth more than a US gallon.
Why two gallons?
Both descend from old English wine and ale gallons. Britain standardised on the imperial gallon in 1824, defined as the volume of 10 pounds of water. The US kept the older Queen Anne wine gallon of 231 cubic inches. The two systems never reconciled, so the gallon split — and the pint and quart split with it.
Every unit, both systems
| Unit | US | UK (imperial) |
|---|---|---|
| 1 gallon | 3.78541 L | 4.54609 L |
| 1 quart | 946.353 ml | 1,136.53 ml |
| 1 pint | 473.176 ml | 568.261 ml |
| 1 cup | 236.588 ml | — (no standard imperial cup) |
| 1 fluid ounce | 29.5735 ml | 28.4131 ml |
Note the twist: the imperial fluid ounce is actually smaller than the US one, even though the imperial gallon is bigger — because the imperial gallon divides into 160 fl oz while the US gallon divides into 128.
What it does to mpg
A car rated 47 mpg in the UK is 47 ÷ 1.201 ≈ 39 mpg in US gallons — the same car, the same fuel, a different gallon. The conversion constants differ too: L/100km = 235.214 ÷ US mpg, but 282.481 ÷ UK mpg. Our mpg to L/100km converter uses US gallons; check which gallon a figure assumes before comparing cars across the Atlantic. The litres to gallons converter lets you pick US or imperial gallons for any volume.
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