US Customary vs Imperial Units: Where They Differ

Same names, different sizes

Anyone who has followed a British recipe in the United States, or the reverse, knows the problem: a pint is a pint, except when it is not. US customary units and British imperial units come from the same family of older English measures, and they share names such as inch, pound, pint and gallon. For length and for ordinary weight, they agree. For volume and for larger weights, they do not.

The split is easier to understand once you know how it arose. Both systems descend from the measures used in England before the nineteenth century. The United States kept the versions it had inherited, while the United Kingdom reformed its standards, and the two drifted apart.

How the split happened

The British reform came with the Weights and Measures Act 1824, effective from 1 January 1826. It created the imperial system and defined the new imperial gallon as the volume of ten pounds of distilled water under set conditions, which gave about 4.5 litres. The United States did not follow. It continued with the older gallon, defined as 231 cubic inches, which is about 3.785 litres.

Later, both countries tied their units to the metric system. The Mendenhall Order of 1893 redefined US units in terms of the metre and kilogram, and the international yard and pound agreement of 1959 fixed the yard at exactly 0.9144 metres and the pound at exactly 0.45359237 kilograms. So today the inch, foot, yard, mile and pound are identical in both systems, but the older differences in volume and large weights remain.

Volume is where most differences appear

The US gallon is 3.785411784 litres. The imperial gallon is exactly 4.54609 litres, roughly 20 per cent larger. Because the pint and quart are fractions of the gallon, the differences carry through. A US liquid pint is about 473.18 millilitres, while an imperial pint is about 568.26 millilitres.

The fluid ounce adds another twist. The US pint has 16 fluid ounces and the imperial pint has 20, but the imperial fluid ounce is slightly smaller: about 28.41 millilitres against roughly 29.57 for the US version, so the US ounce is around 4 per cent larger. This is why “a pint of milk” or “8 fluid ounces” cannot be taken at face value without knowing which system is meant. A US cup is 8 US fluid ounces, about 236.59 millilitres, so a recipe that calls for cups needs to be read with its country of origin in mind.

A worked example with fuel

Suppose a tank holds 5 gallons. If they are US gallons, that is 5 × 3.785411784 = 18.92705892 litres, or about 18.93 litres. If they are imperial gallons, it is 5 × 4.54609 = 22.73045 litres, or about 22.73 litres. The same number, 5, covers a gap of more than 3.8 litres.

This matters in practice when comparing fuel figures or reading old manuals. A car described as doing a certain number of miles per gallon will look more economical in imperial gallons than in US gallons, simply because the imperial gallon is bigger. Always check which gallon a figure uses before comparing two numbers.

Tons and larger weights

The pound is the same in both systems, but the units built on it differ. The US ton, often called the short ton, is 2,000 pounds, which is 907.18474 kilograms. The imperial ton, or long ton, is 2,240 pounds, which is 1,016.0469088 kilograms. Neither is the same as the metric tonne, which is 1,000 kilograms.

The stone, widely used in the United Kingdom and Ireland for body weight, is 14 pounds, or 6.35029318 kilograms. It is a distinctly British unit and is not commonly used in the United States, where body weight is normally given in pounds alone. For example, 10 stone is 140 pounds, or 63.5029318 kilograms.

Other differences worth knowing

The United States still separates liquid and dry measures: a dry pint is not the same as a liquid pint. Imperial measure uses a single gallon for both. The bushel also differs, since the US bushel is based on the older Winchester measure while the imperial bushel is eight imperial gallons.

Surveying had its own quirk. The US survey foot, based on the 1893 definition, differed from the international foot by about two parts per million. The National Institute of Standards and Technology deprecated it after 31 December 2022, apart from historical and legacy uses, so the international foot is now the standard for new work.

Converting safely

The safest habit is to ask which system a number belongs to before converting. Recipes, fuel figures and bottle sizes are the most common traps. A unit converter such as this one lists the US and imperial versions of volume units separately, including the US pint and gallon and the imperial pint and gallon, so you can choose the correct one and see the exact equation.

For medicine, engineering or any safety-related work, check the original source and its stated units rather than assuming. A confusion between pint sizes is harmless in the kitchen, but the same kind of confusion can matter a great deal elsewhere.