Both systems descend from English units, then diverged. The United States kept measures based on the 1707 English wine gallon. Britain reformed its system in 1824, redefining the gallon as the volume of ten pounds of water. The names stayed the same. The sizes did not.
The good news: length and weight are identical
Since the 1959 international yard and pound agreement, these are the same in both systems, to the last decimal:
- Inch — exactly 25.4 mm
- Foot — exactly 0.3048 m
- Yard — exactly 0.9144 m
- Mile — exactly 1,609.344 m
- Pound — exactly 0.45359237 kg
- Ounce (weight) — exactly 28.349523125 g
So "six feet" and "150 pounds" mean the same thing in Manchester and Milwaukee. No conversion needed, no ambiguity.
The bad news: volume is not
| Unit | US customary | Imperial (UK) | Difference |
|---|---|---|---|
| Gallon | 3,785.41 mL | 4,546.09 mL | 20.1% |
| Quart | 946.35 mL | 1,136.52 mL | 20.1% |
| Pint | 473.18 mL | 568.26 mL | 20.1% |
| Cup | 236.59 mL | 284.13 mL | 20.1% |
| Fluid ounce | 29.57 mL | 28.41 mL | 3.9% (US is larger) |
| Tablespoon | 14.79 mL | 17.76 mL | 20.1% |
Note the fluid ounce runs the other way — the US one is larger. That is because the US divides its smaller gallon into 128 fluid ounces while the Imperial gallon is divided into 160. So a UK pint contains 20 fl oz and a US pint contains 16, and the two effects partly cancel.
Recipes. A British recipe calling for a pint of milk wants 568 mL. Using a US measuring cup gives you 473 mL — 17% short. In baking, that is the difference between a batter and a dough.
Fuel economy. "40 miles per gallon" in the UK is 33 mpg US. Comparing a UK review to a US spec sheet without adjusting makes the UK car look 20% more efficient than it is.
Beer. A UK pint is 568 mL; a US pint is 473 mL. Same price, different drink.
How to tell which one you are looking at
- Source country. A UK, Irish or older Commonwealth source means Imperial. A US source means US customary.
- Fluid ounces per pint. If a recipe mentions 20 fl oz to a pint, it is Imperial. 16 means US.
- Explicit labels. Good sources write "US gal" or "imp gal". This site labels every volume unit, which is why the converter's volume dropdown is grouped.
- Date. British recipes from the 1970s onward increasingly use metric. Anything in millilitres and grams sidesteps the problem entirely.
A third system: metric cooking measures
Confusingly, modern UK, Australian, New Zealand and European recipes often use metric spoons and cups, which match neither of the above:
| Unit | Metric | US customary | Imperial |
|---|---|---|---|
| Cup | 250 mL | 236.6 mL | 284.1 mL |
| Tablespoon | 15 mL | 14.8 mL | 17.8 mL |
| Teaspoon | 5 mL | 4.9 mL | 5.9 mL |
The metric cup and the US cup are close enough that swapping them rarely ruins anything — about 5%. The tablespoon is the one to watch, and Australia is the outlier: an Australian tablespoon is 20 mL, a third larger than everywhere else. In a recipe calling for three tablespoons of a strong ingredient, that is a real difference.
Units that exist in only one system
- Stone (14 lb) — UK and Ireland, for body weight. Effectively unused in the US.
- US short ton (2,000 lb) vs UK long ton (2,240 lb) vs metric tonne (1,000 kg = 2,204.6 lb). Three tons within 12% of each other.
- US survey foot — a legacy unit differing from the international foot by 2 parts per million. Irrelevant for a shelf, meaningful across a county boundary. Officially retired at the end of 2022, but it persists in older land records.
The practical rule
For length and weight, convert freely — the systems agree. For anything involving volume, establish the source country first. And when you are writing something others will use, label the unit: "1 pint (US)" costs four characters and removes the whole problem.
Volume units, properly labelled Our converter groups volume into US customary, Imperial and metric cooking measures, so you always know which one you are using.
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