US Gallon vs Imperial Gallon

The word "gallon" names two different units. The US liquid gallon is used in the United States. The imperial gallon comes from the British imperial system. Both are still in use, so a figure in gallons is ambiguous until you know which one is meant.

The imperial gallon is larger. One imperial gallon is about 1.20095 US gallons, and one US gallon is about 0.832674 imperial gallons.

• US liquid gallon: exactly 231 cubic inches. Because the inch is exactly 2.54 cm, this is exactly 3.785411784 liters.
• Imperial gallon: exactly 4.54609 liters. It was originally based on the volume of 10 pounds of water, but today it is defined directly in liters.

Both definitions are exact, so every conversion between them is exact too. The rounded values you see on labels and in tables are just rounding of these exact numbers.

Each system splits its gallon into 4 quarts and 8 pints. The fluid ounce is where they part ways: the US gallon has 128 US fluid ounces, while the imperial gallon has 160 imperial fluid ounces.

• US pint: 16 US fluid ounces.
• Imperial pint: 20 imperial fluid ounces.
• The US fluid ounce is slightly larger than the imperial one: 1 US fl oz ≈ 1.04084 imp fl oz.
• The imperial pint is larger than the US pint: 1 imp pt ≈ 1.20095 US pt.

So the imperial pint has more ounces, and each of its ounces is smaller. The net result is that every imperial unit from the pint up is about one fifth larger than its US namesake.

Miles per gallon depends on which gallon is used. The same car with the same fuel use shows a higher mpg figure in imperial gallons, because each imperial gallon holds more fuel and so lasts more miles.

For example, 30 mpg (US) equals about 36.03 mpg (UK), or 7.84 L/100 km. In the other direction, 50 mpg (UK) is about 41.63 mpg (US).

When you compare cars from US and UK sources, convert both figures to the same unit first. Liters per 100 km is a neutral choice.

The US gallon and its quarts, pints and fluid ounces are used for everyday liquid measures in the United States. The imperial gallon is mainly found in the United Kingdom and in some other countries with a British measurement history, often only in traditional uses such as the pint or mpg figures. Most of these countries sell fuel by the liter.

The United States also has separate dry measures, such as the dry quart and the bushel. They are not the same as the liquid units with similar names.

• A 15 US gallon tank holds 56.7812 L.
• A 15 imperial gallon tank holds 68.1914 L.
• 40 liters is 10.5669 US gallons or 8.79877 imperial gallons.