US Customary vs UK Imperial: Where the Two Systems Actually Differ
"Imperial units" is often used as a catch-all for anything that isn't metric, but American and British measurements aren't actually identical. They share nearly all the same unit names, and for length and weight they mostly agree. Volume is a different story entirely.
Where They Agree: Length and Weight
US customary and UK imperial length units, inches, feet, yards, miles, are effectively identical, both standardized under the 1959 International Yard and Pound Agreement. Weight units, ounces and pounds, also match. The Length and Weight & Mass converters on this site work the same regardless of which country's context you're in.
Where They Split: Volume
Volume is where the two systems genuinely diverge, and the gap is bigger than most people expect.
| Unit | US Customary | UK Imperial |
|---|---|---|
| Fluid ounce | 29.5735295625 mL | 28.4130625 mL |
| Pint | 473.176473 mL (16 fl oz) | 568.26125 mL (20 fl oz) |
| Quart | 0.946352946 L | 1.1365225 L |
| Gallon | 3.785411784 L | 4.54609 L |
The UK imperial gallon is about 20% larger than the US gallon, which is why the same physical fuel tank reports a different capacity depending on which country's gallon is being used, and why UK car fuel-economy figures (miles per imperial gallon) read higher than US figures for an identical car.
The Counterintuitive Part: A US Fluid Ounce Is Bigger
Here's the detail that surprises people: even though the UK gallon is larger overall, the US fluid ounce is about 4.08% larger than the UK imperial fluid ounce. That happens because the US pint holds 16 fluid ounces while the UK pint holds 20, so as the overall gallon size grew in the UK system, the number of fluid ounces per pint grew with it rather than the fluid ounce itself staying fixed. The result: bigger UK pints and gallons, but a slightly smaller UK fluid ounce.
The historical root cause: the US gallon descended from the old English wine gallon (231 cubic inches), while the UK redefined its gallon in 1824 based on the volume of 10 pounds of water, a completely different reference point.
Practical Impact: Recipes, Drinks, and Fuel
A recipe written in "cups" or "pints" from a UK source will measure noticeably more than the same word in a US recipe, roughly 20% more liquid per pint or gallon. Beer and drink serving sizes follow the same split: a UK "pint" of beer is a genuinely larger pour than a US pint. For day-to-day conversion, use the Volume converter's US customary values here, and treat any UK-sourced figure as needing its own separate imperial conversion factor rather than the US one.
Frequently Asked Questions
Does the UK still officially use imperial units?
The UK has been on the metric system since joining what's now the EU, and metric is the legal standard for most trade. In practice, some imperial units persist in everyday British life, road distances in miles, pints for beer and milk, while packaged goods and most other measurements use metric.
Is a UK ton the same as a US ton?
No, and this is a separate divergence from the volume issue: the UK (long) ton is 2,240 lb, while the US (short) ton is 2,000 lb. This site's Weight & Mass converter currently covers pounds, kilograms, grams, ounces, and stone; tons may be added in a future update.