Comparison of the imperial and US customary measurement systems

Both the imperial and United States customary systems of measurement derive from earlier English systems used in the Middle Ages, that were the result of a combination of the local Anglo-Saxon units inherited from German tribes and roman units brought by William the Conqueror after the Norman Conquest of England in 1066.

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 18th century, 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 US–Canada border. It shows equivalents 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) or 4 pecks (8.80976754172 L). 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 dram between their minim and their fluid dram.

One noticeable comparison between the imperial system and the US system is between some Canadian and American beer bottles. Many Canadian brewers package beer in an 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 labelled as 11.5 fl.oz in US units when imported into the United States.

Comparison of current imperial, US and metric volume measures

Notes: Approximate values are denoted with ≈ Exact values are denoted with ≡ Definitions are marked in bold
Unit name	Imperial measures	US fluid measures	US dry measures	Metric measures
fluid ounces
Imperial fluid ounce (fl oz)	≡ 1 fl oz	≈ 0.96075994040 fl oz ≈ 0.94710208333 fl oz (food)		≡ 28.4130625 mL ≡ 0.0284130625 L
US fluid ounce (customary) (fl oz)	≈ 1.04084273079 fl oz	≡ 1 fl oz ≡ 0.98578431875 fl oz (food)		≡ 29.5735295625 mL ≡ 0.0295735295625 L
US fluid ounce (food nutrition labelling) (fl oz) (food)	≈ 1.05585239184 fl oz	≈ 1.01442068106 fl oz ≡ 1 fl oz (food)		≡ 30 mL ≡ 0.03 L
pints
Imperial pint (pt)	≡ 20 fl oz ≡ 1 imp pt ≡ 0.5 imp qt ≡ 0.125 imp gal	≈ 19.2151988081 fl oz ≈ 18.9420416667 fl oz (food) ≈ 1.20094992550 US pt ≈ 0.15011874069 US gal	≈ 1.03205674349 US dry pt ≈ 0.12900709294 US dry gal	≡ 568.26125 mL ≡ 0.56826125 L
US liquid pint (pt)	≈ 16.6534836926 imp fl oz ≈ 0.83267418463 imp pt ≈ 0.10408427308 imp gal	≡ 16 US fl oz ≡ 15.7725491 fl oz (food) ≡ 1 US pt ≡ 0.5 US qt ≡ 0.125 US gal	≈ 0.859367007375 US dry pt ≈ 0.107420875922 US dry gal	≡ 473.176473 mL ≡ 0.473176473 L
US dry pint (pt)	≈ 19.3787794384 imp fl oz ≈ 0.96893897192 imp pt ≈ 0.12111737149 imp gal	≈ 18.6183549784 US fl oz ≈ 18.3536823786 US fl oz (food) ≈ 1.16364718615 US pt ≈ 0.14545589827 US gal	≡ 1 dry pint ≡ 0.5 dry quart ≡ 0.125 dry gallon	≡ 550.6104713575 mL ≡ 0.5506104713575 L
quarts
Imperial quart (qt)	≡ 40 imp fl oz ≡ 1 imp qt ≡ 2 imp pt ≡ 0.25 imp gal	≈ 38.4303976162 US fl oz ≈ 37.8840833333 US fl oz (food) ≈ 1.20094992550 US qt ≈ 0.30023748138 US gal	≈ 1.03205674349 US dry qt ≈ 0.25801418587 US dry gal	≡ 1136.5225 mL ≡ 1.1365225 L
US liquid quart (qt)	≈ 33.3069673852 imp fl oz ≈ 0.83267418463 imp qt ≈ 0.20816854616 imp gal	≡ 32 US fl oz ≡ 31.5450982 US fl oz (food) ≡ 2 US pt ≡ 1 US qt ≡ 0.25 US gal	≈ 0.859367007375 US dry qt ≈ 0.214841751844 US dry gal	≡ 946.352946 mL ≡ 0.946352946 L
US dry quart (qt)	≈ 38.7575588768 imp fl oz ≈ 0.96893897192 imp qt ≈ 0.24223474298 imp gal	≈ 37.2367099567 US fl oz ≈ 36.7073647572 US fl oz (food) ≈ 1.16364718615 US qt ≈ 0.29091179654 US gal	≡ 1 US dry qt ≡ 2 US dry pt ≡ 0.25 US dry gal	≡ 1101.220942715 mL ≡ 1.101220942715 L
gallons
Imperial gallon (gal)	≡ 160 imp fl oz ≡ 4 imp qt ≡ 1 imp gal	≈ 153.721590465 US fl oz ≈ 151.536333333 US fl oz (food) ≈ 1.20094992550 US gal	≈ 4.12822697395 US dry qt ≈ 1.03205674349 US dry gal	≡ 4546.09 mL ≡ 4.54609 L
US liquid gallon (gal)	≈ 133.227869541 imp fl oz ≈ 3.33069673852 imp qt ≈ 0.83267418463 imp gal	≡ 128 US fl oz ≡ 126.1803928 imp fl oz (food) ≡ 4 US qt ≡ 1 US gal ≡ 231 cu inches	≈ 3.437468029501 US dry qt ≈ 0.859367007375 US dry gal	≡ 3785.411784 mL ≡ 3.785411784 L
US dry gallon (gal)	≈ 155.030235507 imp fl oz ≈ 3.87575588768 imp qt ≈ 0.96893897192 imp gal	≈ 148.946839827 US fl oz ≈ 146.829459029 US fl oz (food) ≈ 4.6545887446 US qt ≈ 1.16364718615 US gal	≡ 4 US dry qt ≡ 1 US dry gal ≡ 268.8025 cu inches	≡ 4,404.88377086 mL ≡ 4.40488377086 L
metric
litre (l or L or dm3)	≈ 35.1950797279 imp fl oz ≈ 0.87987699320 imp qt ≈ 0.21996924830 imp gal	≈ 33.8140227018 US fl oz ≈ 33.3333333333 US fl oz (food) ≈ 1.05668820943 US qt ≈ 0.26417205236 US gal	≈ 0.90808298427 US dry qt ≈ 0.22702074607 US dry gal	≡ 1000 mL ≡ 1 L

Length

The international yard is defined as exactly 0.9144 metres. This definition was agreed by the United States, Canada, the United Kingdom, South Africa, Australia and New Zealand in 1959.

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 choice of unit for surveying purposes is based on the unit used in the reference ellipsoid. The ellipsoid is used to set the survey datum points, from which local triangulation is based. The result of this action is that the value is given more precise than the usual six or seven digit conversions, and so older conversions are still retained. A lot of the former British empire still use the Clarke ellipsoid, based on the Clarke foot, (1 metre = 1.09362311 yards).

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 based on the sphere whose surface is the same as the Clarke Ellipsoid. In the US, the full value of 1853.256 metres was used, but in the Commonwealth, this was rounded to 6080 feet (1853.184 m). These have been replaced by the international version, which rounds the sixtieth part of the 45° degree to the nearest metre, as 1852 metres.

Weight and 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 no longer widely 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

Conversion of units English units History of measurement Imperial units

Metric system Metrication SI Systems of measurement

United States customary units Weights and measures Winchester measure