A British recipe calls for a pint of milk; an American cook pours 16 fluid ounces and the sauce comes out thin. A UK car magazine quotes 50 mpg, which is only about 42 mpg in US gallons. Both countries use gallons, pints, ounces and tons, but several of those units have different sizes on each side of the Atlantic.
Why there are two systems
Before 1824, Britain had several gallons in use, for wine, ale and grain. The American colonies inherited the wine gallon of 231 cubic inches and kept it after independence. In 1824 Britain replaced its gallons with a single new imperial gallon, defined by the volume of 10 pounds of water. The two systems have run in parallel ever since.
Today both are fixed in metric terms. The US gallon is exactly 231 cubic inches, which is 3.785411784 liters. The UK (imperial) gallon has been defined since 1985 as exactly 4.54609 liters. Length and weight were settled in 1959, when both countries adopted the same international inch (25.4 mm) and pound (0.45359237 kg). So inches, feet, miles, ounces of weight and pounds are identical in both. Volume and some larger weights are where they differ.
Gallons, quarts and pints
The UK gallon is about 20% larger:
- 1 gal (US) = 3.785411784 L
- 1 gal (UK) = 4.54609 L, which is 1.20095 US gallons
Each gallon divides into 4 quarts and 8 pints in both systems, so the same 20% difference carries all the way down:
- A US pint is 473.176 mL.
- A UK pint is 568.261 mL.
A British pint of beer is that larger 568 mL measure, and it remains the legal serving for draught beer and cider in the UK. Ask for a pint in the US and you’ll usually get 16 US fluid ounces.
The gallons to liters and liters to gallons converters use the US gallon by default and also offer the UK gallon.
The fluid ounce goes the other way
Here’s the odd one. Although the UK pint is larger, the UK fluid ounce is smaller:
- 1 fl oz (US) = 29.5735 mL
- 1 fl oz (UK) = 28.4131 mL
The reason is the number of ounces in a pint. A US pint has 16 fluid ounces; a UK pint has 20. The imperial system divides a larger pint into more, slightly smaller ounces. Bottles labeled in fluid ounces (shampoo, drinks, medicine) therefore don’t mean the same amount in both countries, though most now carry milliliters too. The fluid ounces to milliliters converter covers both.
Cups and spoons
Cups add a third and fourth variant:
- US customary cup: 236.588 mL (8 US fluid ounces), used in most American recipes.
- US legal cup: 240 mL, used on US nutrition labels.
- Metric cup: 250 mL, used in Australia, New Zealand and many Canadian recipes.
- UK: modern British recipes mostly use grams and milliliters rather than cups.
Spoons are closer but not identical. A US tablespoon is 14.787 mL, the metric tablespoon used in the UK, Canada and New Zealand is 15 mL, and an Australian tablespoon is 20 mL (four teaspoons, not three). For most cooking the US and metric tablespoon are interchangeable; the Australian one is about 35% larger, which matters for baking powder and salt.
Because cups measure volume, a cup of flour weighs very different amounts depending on how it’s filled. For baking, weighing is more reliable than any cup. The cups to grams calculator gives weights for common ingredients, and cups to milliliters and tablespoons to teaspoons handle the volume side.
Tons and hundredweights
There are three tons, and they’re all close enough to be confused:
- Short ton (US): 2,000 lb, which is 907.18474 kg.
- Long ton (UK): 2,240 lb, which is 1,016.0469 kg.
- Metric tonne: 1,000 kg, which is 2,204.62 lb.
The long ton survives mainly in shipping and in some historical figures. In the UK and most other countries, “ton” in modern trade usually means the metric tonne, while in the US it means the short ton. Freight quotes, steel prices and vehicle weight limits are worth checking for which one is meant: the difference between a short ton and a long ton is 12%. The US tons to metric tonnes converter handles the American unit.
The hundredweight splits the same way: 100 lb in the US (the short hundredweight) and 112 lb in the UK (the long hundredweight), one-twentieth of each country’s ton.
Stones
The stone is a British and Irish unit with no American equivalent. One stone is 14 pounds, or 6.35029318 kg, and it’s still the everyday way to give body weight in the UK and Ireland: someone weighing 11 stone weighs 154 lb, or about 69.85 kg. The stones to kilograms converter does the arithmetic, including stones and pounds together.
Troy ounces
Gold, silver and other precious metals are priced in troy ounces in both countries. A troy ounce is 31.1034768 grams, about 9.7% heavier than the ordinary (avoirdupois) ounce of 28.3495 grams used for everything else. A “one-ounce” gold coin contains a troy ounce.
Miles per gallon
Fuel economy in miles per gallon depends on which gallon. Because the UK gallon is 20% larger, the same car covers 20% more miles on it:
- UK mpg = US mpg × 1.20095
- A car rated 30 mpg in the US manages about 36 mpg in UK terms.
- A car rated 40 mpg in the UK manages about 33.3 mpg in US terms.
That’s why British fuel economy figures look better than American ones for the same car. To compare across countries, convert to liters per 100 km, which has no such ambiguity: 30 US mpg is 7.84 L/100 km. The mpg to L/100 km converter handles both kinds of mpg.
Quick reference
- Inches, feet, yards, miles, ounces (weight) and pounds: the same in both systems.
- Gallon, quart, pint: UK about 20% larger.
- Fluid ounce: UK about 4% smaller.
- Cup: US 236.6 mL; nutrition labels 240 mL; metric 250 mL.
- Tablespoon: US 14.8 mL; metric 15 mL; Australia 20 mL.
- Ton: US 2,000 lb; UK 2,240 lb; metric 1,000 kg.
- Stone: 14 lb, UK and Ireland only.
When a document doesn’t say which system it uses, look at where it was written and what it’s about. Fuel figures, recipes and beer are the usual places where the difference matters.