US vs UK pints, gallons and fluid ounces: why they differ

US and UK liquid units share names, not sizes. A UK (imperial) pint is 568 ml and a US pint 473 ml, about 20% less, and gallons differ the same way. Yet a US fluid ounce, 29.6 ml, is slightly bigger than a UK one, 28.4 ml; the UK pint just holds 20 of them, not 16.

The figures

• Fluid ounce: US 29.57 ml; imperial 28.41 ml
• Pint: US 473.2 ml (16 US fl oz); imperial 568.3 ml (20 imperial fl oz)
• Quart: US 946.4 ml; imperial 1,136.5 ml
• Gallon: US 3.785 litres; imperial 4.546 litres

The Volume Converter keeps US and imperial units in separate groups for exactly this reason, alongside litres, millilitres and cooking measures.

Why there are two systems

Several different gallons were in use in England before the US became independent, and the US standardised on one of the older ones, the wine gallon of 231 cubic inches. In 1824 Britain replaced its gallons with the new imperial gallon, defined by the volume of 10 pounds of water. The two systems have stayed apart ever since. Both are now defined exactly in metric terms.

Where the difference catches people

• Recipes. An American recipe’s “2 pints of milk” is about 946 ml; a British one is 1,136 ml. For cups, spoons and oven temperatures, see recipe conversions.
• Fuel economy. Miles per gallon figures are not comparable: 40 mpg in the UK is about 33 mpg in US terms, because the UK gallon is bigger. See mpg and litres per 100 km.
• Drinks. A pint of beer in a UK pub is 568 ml; a “pint” glass in the US usually holds about 473 ml.
• Product labels. A US label in fl oz and a UK label in fl oz describe different volumes; the millilitre figure, where shown, is the reliable one.
• Canada and other countries that used imperial measures historically may print either system on older material. Check the source.

Smaller measures

Teaspoons and tablespoons differ too. A US tablespoon is 14.8 ml; a metric tablespoon, used in the UK, Canada and New Zealand, is 15 ml — close enough not to matter. Australia uses a 20 ml tablespoon, which does matter for baking powder or salt. Teaspoons are about 5 ml almost everywhere. Older British recipes may use imperial measures that differ again, so check the age and origin of a recipe if quantities seem odd.

Fluid ounces vs ounces

A fluid ounce measures volume; an ounce measures weight (28.35 g). They match only roughly, and only for water: one imperial fluid ounce of water weighs about one ounce, while a US fluid ounce of water weighs a little more. For flour, sugar or honey they differ a lot, because those are not as dense as water — or are denser. Weigh solids in grams or ounces with the Weight Converter, and measure liquids by volume.

When a label shows both

Products sold in several countries often print a US fl oz figure next to millilitres, even in the UK. A bottle marked “16.9 fl oz / 500 ml” is using US fluid ounces; 500 ml would be 17.6 imperial fluid ounces. Whenever the two figures do not quite agree with the imperial factor, assume the label is American.

Quick conversions to remember

• 1 UK pint ≈ 1.2 US pints; 1 US pint ≈ 0.83 UK pints
• 1 UK gallon ≈ 1.2 US gallons
• 1 litre ≈ 1.76 UK pints ≈ 2.11 US pints
• 1 US cup = 236.6 ml; a “metric cup” = 250 ml

Mistakes to avoid

• Assuming a pint is a pint everywhere.
• Converting UK mpg with a US gallon factor.
• Treating fluid ounces and ounces as the same thing for dry ingredients.

Checklist

• Source American or British (or older Canadian)?
• Volume or weight ounce?
• Millilitre figure available? Use it.