US vs UK Units

Regional · Last updated: July 2025

American and British English share many unit names—gallon, pint, ounce—but the quantities behind those names often differ. This guide clarifies volume, mass, and spelling traps when crossing the Atlantic.

Both the United States and the United Kingdom inherited English customary measures, but they diverged over centuries. The UK consolidated its system into imperial units in 1824. The US kept earlier colonial definitions for several units, especially for volume. Today both countries also use metric officially in science and trade, yet gallons and pints persist on beer taps, milk cartons, and fuel pumps—sometimes with different sizes on each side of the ocean.

Gallons: the biggest everyday difference

The US gallon is defined as exactly 231 cubic inches, about 3.785 liters. The UK (imperial) gallon is about 4.546 liters—roughly 20 percent larger. Fuel economy illustrates the gap: a car averaging 30 miles per US gallon would be about 36 miles per imperial gallon for the same vehicle, because each imperial gallon covers more distance per unit volume.

When a British recipe or document says “1 gallon,” it usually means imperial unless the context is explicitly American (e.g. a US cookbook reprint). Online converters often default to US gallons; always check which unit is selected.

Pints and fluid ounces

A pint of beer in a London pub is 568 mL; a US pint glass is often 473 mL (sometimes advertised as an “imperial pint” in craft bars). Ordering “a pint” in Boston is a smaller pour than in Manchester. Milk in the UK was historically sold in imperial pints; US milk comes in quart and gallon jugs based on the smaller US pint.

Mass ounces and pounds

Avoirdupois ounces and pounds are the same in the US and UK: 1 lb = 16 oz = 0.45359237 kg. The international yard and pound agreement of 1959 harmonized these across English-speaking countries. Where the UK differs is the occasional use of the stone (1 st = 14 lb) for body weight—essentially unknown in the US. Someone saying they weigh “11 stone” is about 154 lb or 70 kg.

Troy ounces (used for precious metals) differ from avoirdupois ounces and are identical in US and UK definitions—do not mix troy and avoirdupois when converting gold or silver weights.

Ton vs tonne

This is a triple trap:

• US short ton = 2,000 lb ≈ 907 kg (common in US industry and shipping)
• UK long ton (weight ton) = 2,240 lb ≈ 1,016 kg (historical UK shipping)
• Metric tonne (t) = 1,000 kg ≈ 2,204 lb (spelled “tonne” to distinguish from tons)

News reports may say “ton” loosely; scientific and EU documents use tonne. A vehicle rated at 3 tons US is not the same mass as 3 tonnes metric. When precision matters, write “short ton,” “long ton,” or “tonne.”

Length: mostly aligned

Inches, feet, yards, and miles match between US customary and UK imperial for trade purposes: 1 in = 2.54 cm exactly; 1 mi = 5,280 ft. Road signs in the UK still show miles and yards; the US uses miles and feet. Nautical miles (1.852 km) are international for aviation and maritime use in both countries.

Paper sizes differ culturally—US Letter vs UK A4—but that is an ISO standard choice, not an imperial vs customary split within length units themselves.

Spelling and vocabulary

British English often uses metre, litre, and gramme while American English prefers meter, liter, and gram. SI symbols (m, L, g) are the same worldwide. UK English may say “petrol” and quote economy in miles per imperial gallon; US English says “gasoline” and miles per US gallon.

Food labeling adds confusion: UK nutrition information is per 100 g; US labels use serving sizes in grams or ounces depending on product category. A “quart” of ice cream in the US is often 1.5 quarts by marketing convention—a packaging quirk, not an imperial redefinition.

When it matters most

Volume conversions bite hardest in cooking, brewing, and automotive contexts. A British cookbook asking for “1 pint of cream” needs imperial pints, not US cups scaled up. Importing a US-spec appliance with gallon-per-minute flow ratings into a UK plumbing discussion requires explicit conversion. Even software localization sometimes hard-codes the wrong gallon constant—developers should expose US vs imperial explicitly in settings.

For mass and length, US and UK agree enough that everyday conversions to metric use the same factors. For volume and tonnage, assume nothing—verify the regional definition first.

Quick conversion reference

Convert US gallons, imperial gallons, pints, and tonnes on the tounits.com converter.