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The (imperial) gallon is an imperial unit of volume.

\(\displaystyle \) \(\) \(\displaystyle 1\) gallon
\(\displaystyle \) \(=\) \(\displaystyle 2\) pottles
\(\displaystyle \) \(=\) \(\displaystyle 4 \cdotp 54609\) litres
\(\displaystyle \) \(=\) \(\displaystyle 4 \, 546 \cdotp 09\) millilitres

Also known as

The imperial gallon is also known as the UK gallon in order specifically to distinguish it from the US gallon.

Also see

Historical Note

Prior to $1824$, there existed a number of different gallons throughout the British Empire.

The imperial gallon was defined in $1824$ as a volume close to the ale gallon.

Its original definition was the volume of $10$ pounds (approximately $4.5$ kilograms) of distilled water, weighed in air with brass weights with the barometer standing at $30$ inches of mercury ($102$ kilopascals) at a temperature of $62$ degrees Fahrenheit ($17$ degrees Celsius).

In $1963$, the imperial gallon was redefined as the volume of $10$ pounds of distilled water of density $0 \cdotp 998859$ grams per millilitre weighed in air of density $0 \cdotp 001217$ grams per millilitre against weights of density $8 \cdotp 136$ grams per millilitre.

This works out to be $4.546096$ litres or $277.4198$ cubic inches.

The imperial gallon was redefined once again in $1985$ to be exactly $4 \cdotp 54609$ litres by the Weights and Measures Act.