What is Imperial gallon?

An Imperial gallon is a volume unit used in the United Kingdom equal to approximately 4.546 litres. It is larger than the US gallon and is commonly referenced for fuel efficiency (miles per gallon) in the UK.

Real-world uses

Imperial gallons are used in the United Kingdom, Canada (for some purposes), and several Caribbean countries. British fuel economy was historically quoted in miles per Imperial gallon. Older Commonwealth-era recipes and industrial specifications may reference Imperial gallons.

History

The Imperial gallon was established by the British Weights and Measures Act of 1824 as the volume occupied by 10 pounds of water at 62°F (≈ 17°C). It replaced several older gallon definitions including the wine gallon (retained by the US) and the ale gallon.

Common mistakes

Assuming the Imperial gallon equals the US gallon. The Imperial gallon is about 4.546 litres versus 3.785 litres for the US gallon. Converting mpg figures between the two systems without adjusting for gallon size gives misleading results.

What is US gallon?

A US gallon is a volume unit equal to 128 US fluid ounces, or approximately 3.785 litres. It is the standard unit for fuel, milk, and other liquid products in the United States.

Real-world uses

US gallons are used for fuel pricing and vehicle fuel economy (miles per gallon) in the United States. Milk, water, and paint are commonly sold in gallon containers. Pool volumes and water heater capacities are specified in gallons.

History

The US gallon is based on the English wine gallon of 231 cubic inches, which was defined by Queen Anne in 1707. When Britain adopted the Imperial gallon in 1824 (based on water weight), the US retained the older wine gallon standard.

Common mistakes

Confusing US gallons with Imperial gallons: a US gallon is about 3.785 litres while an Imperial gallon is about 4.546 litres—roughly 20% larger. This causes major confusion when comparing fuel economy between US and UK figures.

When is this conversion used?

Fuel economy, beverage volumes, and liquid measurements frequently require converting between litres and US gallons, especially for automotive and cooking contexts across different markets.

Worked examples

1 Imperial gallon = 1.20094993 US gallon

1 US gallon = 3.78541178 litre

How to convert Imperial gallon to US gallon

To convert Imperial gallon to US gallon, multiply the value by 1.20094993.

To convert US gallon back to Imperial gallon, multiply by 0.83267418.

Measurement standards

The cubic metre is the SI derived unit of volume. The litre, equal to exactly one cubic decimetre (0.001 m³), is accepted by the BIPM for use alongside SI units. Both the lowercase "l" and uppercase "L" are approved symbols for the litre.

Did you know?

An Olympic swimming pool holds 2,500 cubic metres of water — about 2.5 million litres. The Amazon River discharges roughly 209,000 cubic metres per second, enough to fill 84 Olympic pools every single second.

Quick reference: Imperial gallon to US gallon

Imperial gallonUS gallon
0.10.12009499
0.50.60047496
11.20094993
22.40189985
56.00474963
1012.00949926
2530.02374814
5060.04749628
100120.09499255
250300.23748138
500600.47496275
1,0001,200.9499255

Common values

Imperial gallonUS gallon
A teaspoon0.00109985 Imperial gallon0.00132086 US gallon
A cup of coffee0.05499231 Imperial gallon0.06604301 US gallon
A water bottle0.10998462 Imperial gallon0.13208603 US gallon
A bathtub65.99077449 Imperial gallon79.25161571 US gallon
A swimming pool (Olympic)549,923.12074772 Imperial gallon660,430.13089537 US gallon

Available Volume units

More Imperial gallon conversions