Comparison of the imperial and US customary measurement systems

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Both the imperial and United States customary systems of measurement derive from earlier English systems developed in England since the Battle of Hastings in 1066. They were a combination of the Anglo-Saxon and Roman systems.

Having this shared heritage, the two systems are quite similar, but there are differences. The US customary system is based on English systems of the 1700s, while the Imperial system was defined in 1824, after American independence.

Volume

A baby bottle that measures in all three measurement systems—metric, imperial (UK), and US customary.

A one US gallon gas can purchased near the USA-Canada border. It shows equivalences in imperial gallons and litres.

The imperial gallon was originally defined as the volume of 10 avoirdupois pounds of water under specified conditions. The imperial gallon (4.54609 litres (L)) is 20% larger than the United States liquid gallon (3.785411784 L). The imperial bushel (36.36872 L) is 8 imperial gallons and is about 3% larger than the US bushel (35.23907016688 L)

The subdivision of the imperial gallon in British apothecaries' fluid measure differed in two important respects from the corresponding United States subdivision: the imperial gallon was divided into 160 fluid ounces, the United States gallon into 128 fluid ounces; and a "fluid scruple" is included.

These differences come from the various systems that were in use in Britain when the first colonies in North America were established. The American colonists adopted the English wine gallon of 231 cubic inches (3.78541178 litres), and used it for all fluid purposes. The English of that period used this wine gallon, but they also had the ale gallon of 282 cubic inches (4.62115205 litres). In 1824, the British adopted the British imperial gallon, defined as the volume of 10 pounds of water at a temperature of 62 °F, weighed in air with brass weights, by calculation equivalent to about 277.42 cubic inches (4.5461 L)—much closer to the ale gallon than the wine gallon. At the same time, they redefined the bushel to be 8 gallons.

Even under the new imperial system, wine volumes continued to be measured in the old 231-cubic-inch wine gallons (3.78541178 litres) for tax purposes, and this practice continued until the late 1990s.

As noted above, in the imperial British system the units of dry measure are the same as those of liquid measure. In the United States these two are different: the gallon and its subdivisions are used in the measurement of liquids, the bushel and its subdivisions in the measurement of certain dry commodities. The US gallon (3.785411784 L) is divided into four liquid quarts (946.352946 mL each) and the US bushel (35.23907016688 L) into 32 dry quarts (1.101220942715 L each) or 4 pecks (8.80976754172 L each). All these units of volume for liquid measures are about 20% larger in the imperial system than in the US system. However, the British fluid ounce is only about 96% of the US fluid ounce because there are 40 fluid ounces in the British quart but only 32 fluid ounces in the US quart.

In the imperial system an avoirdupois ounce of water at 62 °F (16.67 °C) has a volume of one fluid ounce, because 10 pounds is equivalent to 160 avoirdupois ounces and 1 imperial gallon is equivalent to 4 imperial quarts, or 8 pints. This convenient fluid-ounce-to-avoirdupois-ounce relation does not exist in the US system because a US gallon of water at 62 °F weighs about 8+¹⁄₃ pounds, or 133+¹⁄₃ avoirdupois ounces, and the US gallon is equivalent to 128 fluid ounces.

In the apothecary system of liquid measure the British add a unit, the fluid scruple, equal to one third of a fluid drachm (spelt dram in the United States) between their minim and their fluid drachm.

One noticeable comparison between the imperial system and the US system is between some Canadian and American beer bottles. Many Canadian brewers, like Labatt, package beer in 12-imperial-fluid-ounce bottles, which are 341 mL each. American brewers package their beer in 12-US-fluid-ounce bottle, which are 355 mL each. This results in the Canadian bottles being labeled as 11.5 fl.oz in US units when imported into the United States.

Length

The international yard is defined as exactly 0.9144 metres. This definition was agreed by the US, Canada, the UK, South Africa, Australia and New Zealand in 1959. However, the US continued to use its previous national definition of the length units for surveying purposes.

The US survey foot is defined so that 1 metre is exactly 39.37 inches; the international foot is exactly two parts per million shorter. The US survey foot and survey mile have been maintained as separate units for surveying purposes. This was done to avoid the accumulation of error that would follow replacing them with the international versions. This was not a problem for the United Kingdom, as the Ordnance Survey has been metric since before World War II.

The main units of length (inch, foot, yard and international mile) were the same in the USA, though some of the intermediate units, such as the (surveyor's) chain (22 yards) and the furlong (220 yards), were hardly used there.

At one time the nautical mile was defined differently in the UK and the US, but today both countries use the international definition of 1852 metres.

Mass

Both Britain and the US traditionally used three different weight systems. There was troy weight for precious metals and avoirdupois weight for most other purposes. The third, apothecaries' weight, has been superseded by the metric system.

One important difference is the widespread use in Britain of the stone of 14 pounds (6.35029318 kg) for body weight. This unit is not used in the United States, although its influence was seen in the practice until World War II of selling flour by a barrel of 196 pounds (14 stone). Another difference arose when Britain abolished the troy pound (373.2417216 g) on January 6, 1879, leaving only the troy ounce (31.1034768 g) and its decimal subdivisions, whereas the troy pound (of 12 troy ounces) and pennyweight are still legal in the United States, although they are not now greatly used.

In all the systems, the fundamental unit is the pound (lb), and all other units are defined as fractions or multiples of it. The tables of imperial troy mass and apothecaries' mass are the same as the corresponding United States tables, except for the British spelling "drachm" in the table of apothecaries' mass. The table of imperial avoirdupois mass is the same as the United States table up to 1 pound, but above that point the tables differ.

The imperial system uses a hundredweight of eight stone or 112 lb (50.80234544 kg), whereas a US hundredweight is 100 lb (45.359237 kg). In both systems, 20 hundredweights make a ton. In the US, the terms long ton (2240 lb, 1016.0469088 kg) and short ton (2000 lb; 907.18474 kg) are used to distinguish them. The term metric ton is also used to denote a tonne (1000 kg, 2204.622 lb), which is about 2% less than the long ton.

See also