UK Gallons vs US Gallons: Why the Litre Conversion Differs

If you have ever converted a recipe, compared a car’s fuel economy across the Atlantic, or ordered a pint in two countries and noticed the difference, you have run into one of the oddest survivals in everyday measurement: there are two gallons, and they are not the same size.

An imperial (UK) gallon is 4.54609 litres. A US liquid gallon is 3.785411784 litres. That is a gap of roughly 20 percent, which is more than enough to ruin a fuel-economy comparison or a batch of stock. This guide explains exactly how big the difference is, why it exists, and how to convert in either direction without getting it backwards.

The short answer, in numbers

Both gallons are now defined exactly, so these are not rounded estimates. The imperial gallon has been legally fixed at exactly 4.54609 litres in the UK since the Weights and Measures Act 1985. The US liquid gallon is defined as exactly 231 cubic inches, and because the inch is exactly 2.54 centimetres, that works out to exactly 3.785411784 litres.

Unit	Imperial (UK)	US customary	Imperial as a ratio of US
Gallon	4.54609 L	3.785411784 L	1.2009
Quart	1.13652 L	0.94635 L	1.2009
Pint	568.26 mL	473.18 mL	1.2009
Fluid ounce	28.413 mL	29.574 mL	0.9608
Fluid ounces per pint	20	16	—

Two numbers are worth memorising. An imperial gallon is about 1.2 US gallons, and a US gallon is about 0.833 imperial gallons. Put the other way round: the US gallon is roughly 16.7 percent smaller than the imperial one, while the imperial gallon is roughly 20.1 percent larger than the US one. Those two percentages describe the same gap from opposite ends, which is a common source of confusion on its own.

Why there are two gallons at all

England spent centuries with several gallons running side by side: a wine gallon, an ale gallon and a corn gallon, each a different size because each trade had its own standard vessel. The wine gallon was fixed by statute in 1707 at 231 cubic inches. The United States gained independence while that wine gallon was still the working standard for liquids, kept it, and never changed it. That is why the US liquid gallon is still exactly 231 cubic inches today.

Britain then went the other way. The Weights and Measures Act 1824 swept away the competing gallons and created a single imperial gallon, defined as the volume of ten pounds of distilled water under specified conditions of temperature and air pressure, which came to about 277.42 cubic inches. The 1985 Act replaced that water-based definition with the flat metric figure of 4.54609 litres.

So the difference is not a rounding error or a metric-versus-imperial quirk. It is two countries preserving two different historical vessels. The US also retains a separate dry gallon of 268.8025 cubic inches, about 4.40488 litres, for grain and produce. It is rare, but if a figure matches neither number above, a dry measure is often the reason.

How to convert, step by step

The safest method is always to go through litres rather than trying to apply the 1.2 ratio directly. Litres act as a neutral middle step, so you cannot accidentally multiply when you should divide.

1. Identify which gallon you have. A US source, an American recipe, a US fuel-economy rating or anything quoted by the EPA means US gallons. A UK source, a DfT or manufacturer mpg figure for the British market, or an older British recipe means imperial gallons.
2. Convert to litres. Multiply US gallons by 3.785412, or imperial gallons by 4.54609.
3. Convert out of litres. Divide litres by 4.54609 for imperial gallons, or by 3.785412 for US gallons.
4. Sanity-check the direction. Going from US gallons to imperial gallons, the number must get smaller. Going from imperial to US, it must get bigger. If your answer moved the wrong way, you have divided where you should have multiplied.

For fuel economy the logic inverts, and this catches almost everyone. Miles per gallon rises when the gallon gets bigger, because you are covering the same miles on a larger unit of fuel. So a US mpg figure becomes a larger imperial mpg figure: multiply by 1.2009. A UK mpg figure becomes a smaller US mpg figure: divide by 1.2009.

Worked example: an American car on a British forecourt

Suppose a car is advertised in the United States at 32 mpg combined, and you want to know what that means in the UK, where fuel is sold by the litre and economy is quoted in imperial mpg.

Step 1 — convert the economy figure.
32 US mpg times 1.2009 = 38.4 imperial mpg.

Step 2 — cross-check through litres per 100 km.
From US mpg: 235.215 divided by 32 = 7.35 L/100 km.
From imperial mpg: 282.481 divided by 38.4 = 7.35 L/100 km.
Both routes agree, which confirms the conversion is right. Each constant is simply 100 times the gallon size in litres divided by 1.609344 km per mile.

Step 3 — size the tank.
Say the car has a 50 litre tank.
50 divided by 4.54609 = 11.00 imperial gallons.
50 divided by 3.785412 = 13.21 US gallons.
The same physical tank, two very different gallon counts.

Step 4 — put a price on it.
Take an illustrative UK pump price of £1.40 per litre. A full 50 litre tank costs 50 times £1.40 = £70.00.
Per imperial gallon: £70.00 divided by 11.00 = £6.36.
Per US gallon: £70.00 divided by 13.21 = £5.30.

Step 5 — compare with a US pump price.
Take an illustrative US price of $3.20 per US gallon.
Per litre: $3.20 divided by 3.785412 = $0.845.
At an example rate of $1.30 to the pound: $0.845 divided by 1.30 = £0.65 per litre.
At $1.10 to the euro, that same litre is about €0.77.

The figures above are illustrative and used only to show the method; pump prices and exchange rates move constantly, so substitute the numbers you are actually looking at.

Pints, fluid ounces and other places this bites

The pint follows the gallon, so a UK pint is 568 mL and a US pint is 473 mL. That is why a pint of beer or a pint of milk feels noticeably more generous in Britain. Under UK law the imperial pint remains a permitted unit for draught beer and cider and for milk in returnable containers, even though fuel has been sold in litres since the mid-1990s.

The fluid ounce is the genuine trap. The imperial fluid ounce is 28.413 mL and the US fluid ounce is 29.574 mL, so the US one is larger, despite the US gallon being smaller. The reason is the division: an imperial pint holds 20 fluid ounces while a US pint holds only 16. A recipe calling for 8 fl oz of liquid therefore means about 237 mL in an American cookbook and about 227 mL in a British one. That gap of roughly 9 mL rarely matters in a stew and matters a great deal in baking or in a cocktail. If you work with recipes often, our cooking measurement converter handles cups, ounces and millilitres side by side.

One more figure worth knowing: the oil barrel is 42 US gallons, or 158.99 litres. Commodity reporting uses barrels universally, so a barrel figure always sits on the US side of this divide.

Frequently asked questions

Is a US gallon bigger or smaller than a UK gallon?

Smaller. A US liquid gallon is 3.785412 litres against the imperial gallon’s 4.54609 litres, so it holds about 83.3 percent as much, or about 16.7 percent less. Stated the other way, an imperial gallon holds about 20.1 percent more than a US gallon.

Why is a US fluid ounce larger if the US gallon is smaller?

Because the two systems cut the pint into a different number of pieces. An imperial pint contains 20 fluid ounces and a US pint contains 16. Dividing the smaller US pint into fewer parts produces a slightly larger ounce: 29.574 mL against 28.413 mL. The gallons differ by about 20 percent, but the fluid ounces differ by only about 4 percent, in the opposite direction.

Does the UK still use gallons?

Not for selling fuel. Petrol and diesel have been sold by the litre in the UK since the mid-1990s, and litres are the legal unit at the pump. Gallons survive mainly in fuel-economy figures, where manufacturers and the Department for Transport still quote miles per imperial gallon, and in informal conversation. When a UK mpg figure looks generous next to an American one for the same car, the larger gallon is usually the whole explanation.

Convert it in one step

The arithmetic is simple once you know which gallon you are holding, but it is easy to apply the ratio the wrong way round when you are juggling recipes, fuel figures and tank sizes in one sitting. Our free volume converter handles imperial gallons, US liquid gallons, litres, pints, quarts and fluid ounces in both directions, so you can check a figure rather than trust a half-remembered multiplier. For everything from length to weight, the full set of unit converters is free with no sign-up.