US Customary vs Imperial Units
The US and UK share unit names, but not always unit sizes. Knowing which ones differ avoids expensive mistakes.
Units that are the same
Since 1959 both systems use the same inch (2.54 cm), foot, yard and mile, and the same avoirdupois pound (0.45359237 kg) and ounce. So lengths, and weights up to the pound, convert identically.
Units that differ
| Unit | US | Imperial (UK) | Difference |
|---|---|---|---|
| Fluid ounce | 29.574 mL | 28.413 mL | US is 4% larger |
| Pint | 473.18 mL (16 fl oz) | 568.26 mL (20 fl oz) | UK is 20% larger |
| Gallon | 3.7854 L | 4.5461 L | UK is 20% larger |
| Ton | 2,000 lb (short ton) | 2,240 lb (long ton) | UK is 12% larger |
| Hundredweight | 100 lb | 112 lb | — |
| Stone | not used | 14 lb | — |
Practical consequences
- Fuel economy: 30 mpg (US) is 36 mpg (UK) for the same car, because the UK gallon is larger.
- A “pint” of beer in a British pub is 568 mL; in the US, 473 mL.
- Recipes: a US cup (236.6 mL) differs from the metric cup (250 mL) used in Australia, while the UK mostly weighs ingredients.