US vs Imperial Volume

Frequently asked questions

Is an imperial gallon bigger than a US gallon?

Yes. An imperial (UK) gallon is 4.54609 L, versus 3.78541 L for a US gallon — the imperial gallon is about 20.1% larger. The same 1.20095× ratio applies to pints and quarts, since both systems divide their gallon into quarts and pints the same way (1 gal = 4 qt = 8 pt).

Is an imperial fluid ounce bigger or smaller than a US fluid ounce?

Smaller — an imperial fluid ounce is about 3.9% smaller than a US fluid ounce, even though the imperial gallon itself is larger. That's because a US gallon is defined as 128 US fluid ounces, while an imperial gallon is defined as 160 imperial fluid ounces — dividing a bigger gallon into more, smaller pieces.

Why doesn't the fluid ounce follow the same ratio as the gallon?

Fluid ounces aren't a fixed physical volume shared by both systems — each system defines its own fluid ounce as a fraction of its own gallon. The US gallon splits into 128 fluid ounces; the imperial gallon splits into 160. Pints, quarts, and gallons all split the same way (4 quarts and 8 pints per gallon) in both systems, so they scale together — fluid ounces don't.

Does the UK still use imperial pints and gallons?

The UK uses metric units (liters, milliliters) for most packaged goods, but the imperial pint remains the legal unit for draught beer, cider, and milk in returnable bottles, and fuel is sometimes still discussed in miles per imperial gallon.