Where the systems agree
Length and mass are identical on both sides of the Atlantic, because both countries now define them from the metric standards. The UK Weights and Measures Act 1985 sets the yard at 0.9144 metre and the pound at 0.453 592 37 kilogram; NIST's conversion tables give the same figures and mark the inch, at 0.0254 metre, and the mile, at 1,609.344 metres, as exact. So an inch, a foot, a yard, a mile, an ounce and a pound mean the same thing in a US workshop and a British one, and the length converter and weight converter need no country setting for them.
Where they part: volume
The two gallons are different sizes, and every smaller liquid unit inherits the difference:
| Unit | US customary | Imperial (UK) | Imperial compared with US |
|---|---|---|---|
| Fluid ounce | 29.57 mL | 28.41 mL | 3.9% smaller |
| Pint | 473.2 mL (16 US fl oz) | 568.3 mL (20 imperial fl oz) | 20.1% larger |
| Quart | 0.946 L | 1.137 L | 20.1% larger |
| Gallon | 3.785 L | 4.546 L | 20.1% larger |
Two facts explain the whole table. The US gallon is 3.785 litres and the imperial gallon 4.546 litres, by NIST's factors and the UK Act respectively, so the imperial gallon, quart and pint are each about 20 percent larger. But the imperial pint is divided into 20 fluid ounces where the US pint has 16, so the imperial fluid ounce ends up slightly smaller than the US one. A recipe in fluid ounces converts almost unchanged; a recipe in pints or cups does not. The volume converter lists both gallons and both fluid ounces as separate units, and the cooking measurement converter handles cups, which are a US kitchen unit of 8 US fluid ounces; the fluid ounces guide covers the further confusion between fluid and weight ounces.
Weight: the same pound, different multiples
| Unit | Definition |
|---|---|
| Ounce | 1/16 pound, 28.35 g |
| Pound | 0.45359237 kg exactly, in both systems |
| Stone | 14 pounds (UK only; not used in the US) |
| Hundredweight | UK: 112 lb. US: 100 lb |
| Ton | UK long ton: 2,240 lb = 1016.0 kg. US short ton: 2,000 lb = 907.2 kg |
The pound and ounce match. Above the pound the two traditions diverge: the UK Act defines the stone as 14 pounds, the hundredweight as 112 and the ton as 2,240, while US practice uses a 100-pound hundredweight and the 2,000-pound short ton. NIST lists both tons with their metric values, 907 kg short and 1016 kg long, next to the metric tonne of 1,000 kg. Body weight in the UK is commonly given in stone and pounds, 11 st 4 lb, which the weight converter reads directly.
Where it goes wrong
- Fuel economy. Miles per gallon in a British car magazine uses the larger gallon, so the same car scores about 20 percent more mpg in the UK than in the US. Compare in litres per 100 km instead.
- Recipes. A British pint of milk is 568 mL, an American pint 473 mL. The cup is a US unit; British recipes more often give weights or millilitres.
- Freight and bulk goods. "Ton" needs a qualifier: short, long or metric. The three differ by up to 12 percent.
- Fluid ounces on packaging. A bottle labelled in fl oz means US fluid ounces on a US label and imperial on a UK one; the millilitre figure beside it is the one to trust.
The unit converter keeps the US and imperial variants as distinct entries for exactly these cases; the rule is to convert through the metric figure printed on the label or in the standard, which both systems are defined against.
Frequently asked questions
Are US and imperial units the same?
Length and weight, yes: the inch, foot, yard, mile, ounce and pound are identical, both now defined from the metre and the kilogram. Volume, no: the US gallon is 3.785 litres and the imperial gallon 4.546 litres, and the pints, quarts and fluid ounces differ with them.
Why is a UK pint bigger than a US pint?
Because the gallons differ and the pint is an eighth of each. A US pint is 16 US fluid ounces, about 473 mL; an imperial pint is 20 imperial fluid ounces, about 568 mL, as the UK Weights and Measures Act defines it. A pint of beer in London is a fifth larger than one in New York.
Is a US fluid ounce the same as an imperial fluid ounce?
Nearly, but the other way round from the pint: the US fluid ounce is 29.57 mL and the imperial one 28.41 mL, so the US ounce is about 4 percent larger while the US pint is about 17 percent smaller. The difference comes from 16 versus 20 ounces to the pint.
What is a stone?
Fourteen pounds, a UK unit for body weight that the Weights and Measures Act 1985 lists among the imperial units. Eleven stone is 154 pounds or about 69.9 kilograms. It is not used in the United States, where body weight is given in pounds alone.
How many pounds in a ton?
It depends which ton. The US short ton is 2,000 pounds, about 907 kg; the UK long ton is 2,240 pounds, about 1,016 kg; the metric tonne is 1,000 kg, about 2,205 pounds. NIST lists all three, and a shipping or freight quote should say which it means.
Sources
- NIST Special Publication 811, Appendix B.9: Factors for units listed by kind of quantity (gallon U.S. 3.785 412 L; gallon U.K. imperial 4.546 09 L; fluid ounce U.S. 29.573 53 mL and U.K. 28.413 06 mL; pint U.S. liquid 0.473 176 5 L; pound 0.453 592 4 kg; ton short 907.184 7 kg and long 1 016.047 kg; mile 1 609.344 m and inch 0.0254 m exact)
- UK Weights and Measures Act 1985, Schedule 1: definitions of units of measurement (yard 0.9144 metre; mile 1,760 yards; gallon 4.54609 cubic decimetres; quart ¼ gallon; pint 0.568 261 25 cubic decimetre; fluid ounce 1/20 pint; pound 0.453 592 37 kilogram; ounce 1/16 pound; stone 14 pounds; hundredweight 112 pounds; ton 2,240 pounds)