Why the US Gallon Is Smaller Than the Imperial Gallon

Before 1824 England had several gallons at once: a wine gallon of 231 cubic inches, an ale gallon of about 282 cubic inches and a corn gallon for dry goods. The American colonies inherited this jumble, and after independence the United States settled on the 231 cubic inch wine gallon for liquids. It is defined today as exactly 3.785411784 litres.

Britain went the other way. The Weights and Measures Act 1824 swept away the old gallons and created the imperial gallon, the volume of ten pounds of distilled water weighed in air under specified conditions. The Weights and Measures Act 1985 fixed it at exactly 4.54609 litres, about 20 per cent more than the American gallon.

The difference ripples through every smaller unit. An imperial pint is 20 imperial fluid ounces, 568.26125 mL; a US pint is 16 US fluid ounces, 473.176473 mL. Yet the US fluid ounce, at 29.5735295625 mL, is slightly larger than the imperial one at 28.4130625 mL, because the US gallon is split into 128 fluid ounces and the imperial gallon into 160.

The practical consequences are familiar to anyone comparing cars or recipes. A car that does 40 miles per imperial gallon does only about 33 miles per US gallon, and a British pint of beer is about a fifth larger than an American one. Dry goods add another layer, since the US bushel follows the old Winchester bushel of 2150.42 cubic inches rather than eight imperial gallons.