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Comparison of the imperial and US customary measurement systems

For a topical guide to this subject, see 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

File:MetricImperialUSCustomaryUnits.jpg
A baby bottle that measures in all three measurement systems—metric, imperial (UK), and US customary.
File:GasCan.jpg
A one-US-gallon gas can purchased near the US–Canada border. It shows equivalents in imperial gallons and litres.

Volume may be measured either in terms of units of cubic length or with specific volume units. The units of cubic length (the cubic inch, cubic foot, cubic mile, etc.) are the same in the imperial and US customary systems but with the specific units of volume (the bushel, gallon, fluid ounce, etc.) they differ. The US customary system has one set of units for fluids and another set for dry goods. The imperial system has only one set defined independently of and subdivided differently from its US counterparts.

By the end of the eighteenth century various systems of volume measurement were in use throughout the British Empire. Wine was measured with units based on the Queen Anne's gallon of 231 cubic inches (3.785 L). Beer was measured with units based on an ale gallon of 282 cubic inches (4.621 L). Grain was measured with the Winchester measure with a gallon of approximately 268.8 cubic inches (one eighth of a Winchester bushel or 4.405 L). In 1824 these were replaced with a single system based on the imperial gallon.[note 1] Originally defined as the volume of Script error: No such module "convert". of distilled water (under certain conditions),[note 2] then redefined by The Weights and Measures Act of 1985 to be exactly 4.54609Lua error: Unmatched close-bracket at pattern character 67. (277.4 cu in), the imperial gallon is close in size to the old ale gallon.

The Winchester measure was made obsolete in the British Empire but remained in use in the US.[note 3] The Winchester bushel was replaced with an imperial bushel of 8 imperial gallons. The subdivisions of the bushel were maintained. As with US dry measures the imperial system divides the bushel into 4 pecks, 8 gallons, 32 quarts or 64 pints. Thus all of these imperial measures are about 3% larger than their US dry measure counterparts.

Fluid measure is not as straightforward. The American colonists adopted a system based on the 231-cubic-inch wine gallon for all fluid purposes. This became the US fluid gallon. Both the imperial and US fluid gallon are divided into 4 quarts, 8 pints or 32 gills.[note 4] However, whereas the US gill is divided into 4 US fluid ounces, the imperial gill is divided into 5 imperial fluid ounces. So whilst the imperial gallon, quart, pint and gill are about 20% larger than their US fluid measure counterparts, the fluid ounce is about 4% smaller.[note 5] Note that one avoirdupois ounce of water has an approximate volume of one imperial fluid ounce at 62 °F (16.67 °C).[note 6] This convenient fluid-ounce-to-avoirdupois-ounce relation does not exist in the US system.

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. Because Canadian beer bottles predate the adoption of the Metric System in that country, they are still sold and labelled in Canada as 341 mL. Canned beer in Canada is sold and labelled in 355 mL cans, and when exported to the US are labelled as 12 fl oz.

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)

1Lua error: Unmatched close-bracket at pattern character 67.

0.96075994040Lua error: Unmatched close-bracket at pattern character 67.
0.94710208333Lua error: Unmatched close-bracket at pattern character 67. (food)

28.4130625Lua error: Unmatched close-bracket at pattern character 67.
0.0284130625Lua error: Unmatched close-bracket at pattern character 67.

US fluid ounce
(customary)
(fl oz)

1.04084273079Lua error: Unmatched close-bracket at pattern character 67.

1Lua error: Unmatched close-bracket at pattern character 67.
0.98578431875Lua error: Unmatched close-bracket at pattern character 67. (food)

29.5735295625Lua error: Unmatched close-bracket at pattern character 67.
0.0295735295625Lua error: Unmatched close-bracket at pattern character 67.

US fluid ounce
(food nutrition labelling)
(fl oz) (food)

1.05585239184Lua error: Unmatched close-bracket at pattern character 67.

1.01442068106Lua error: Unmatched close-bracket at pattern character 67.
1Lua error: Unmatched close-bracket at pattern character 67. (food)

30Lua error: Unmatched close-bracket at pattern character 67.
0.03Lua error: Unmatched close-bracket at pattern character 67.


pints
Imperial pint
(pt)

20Lua error: Unmatched close-bracket at pattern character 67.
1Lua error: Unmatched close-bracket at pattern character 67.
0.5Lua error: Unmatched close-bracket at pattern character 67.
0.125Lua error: Unmatched close-bracket at pattern character 67.

19.2151988081Lua error: Unmatched close-bracket at pattern character 67.
18.9420416667Lua error: Unmatched close-bracket at pattern character 67. (food)
1.20094992550Lua error: Unmatched close-bracket at pattern character 67.
0.15011874069Lua error: Unmatched close-bracket at pattern character 67.

1.03205674349Lua error: Unmatched close-bracket at pattern character 67.
0.12900709294Lua error: Unmatched close-bracket at pattern character 67.

568.26125Lua error: Unmatched close-bracket at pattern character 67.
0.56826125Lua error: Unmatched close-bracket at pattern character 67.

US liquid pint
(pt)

16.6534836926Lua error: Unmatched close-bracket at pattern character 67.
0.83267418463Lua error: Unmatched close-bracket at pattern character 67.
0.10408427308Lua error: Unmatched close-bracket at pattern character 67.

16Lua error: Unmatched close-bracket at pattern character 67.
15.7725491Lua error: Unmatched close-bracket at pattern character 67. (food)
1Lua error: Unmatched close-bracket at pattern character 67.
0.5Lua error: Unmatched close-bracket at pattern character 67.
0.125Lua error: Unmatched close-bracket at pattern character 67.

0.859367007375Lua error: Unmatched close-bracket at pattern character 67.
0.107420875922Lua error: Unmatched close-bracket at pattern character 67.

473.176473Lua error: Unmatched close-bracket at pattern character 67.
0.473176473Lua error: Unmatched close-bracket at pattern character 67.

US dry pint
(pt)

19.3787794384Lua error: Unmatched close-bracket at pattern character 67.
0.96893897192Lua error: Unmatched close-bracket at pattern character 67.
0.12111737149Lua error: Unmatched close-bracket at pattern character 67.

18.6183549784Lua error: Unmatched close-bracket at pattern character 67.
18.3536823786Lua error: Unmatched close-bracket at pattern character 67. (food)
1.16364718615Lua error: Unmatched close-bracket at pattern character 67.
0.14545589827Lua error: Unmatched close-bracket at pattern character 67.

1 dry pint
≡ 0.5 dry quart
≡ 0.125 dry gallon

550.6104713575Lua error: Unmatched close-bracket at pattern character 67.
0.5506104713575Lua error: Unmatched close-bracket at pattern character 67.


quarts
Imperial quart
(qt)

40Lua error: Unmatched close-bracket at pattern character 67.
1Lua error: Unmatched close-bracket at pattern character 67.
2Lua error: Unmatched close-bracket at pattern character 67.
0.25Lua error: Unmatched close-bracket at pattern character 67.

38.4303976162Lua error: Unmatched close-bracket at pattern character 67.
37.8840833333Lua error: Unmatched close-bracket at pattern character 67. (food)
1.20094992550Lua error: Unmatched close-bracket at pattern character 67.
0.30023748138Lua error: Unmatched close-bracket at pattern character 67.

1.03205674349Lua error: Unmatched close-bracket at pattern character 67.
0.25801418587Lua error: Unmatched close-bracket at pattern character 67.

1136.5225Lua error: Unmatched close-bracket at pattern character 67.
1.1365225Lua error: Unmatched close-bracket at pattern character 67.

US liquid quart
(qt)

33.3069673852Lua error: Unmatched close-bracket at pattern character 67.
0.83267418463Lua error: Unmatched close-bracket at pattern character 67.
0.20816854616Lua error: Unmatched close-bracket at pattern character 67.

32Lua error: Unmatched close-bracket at pattern character 67.
31.5450982Lua error: Unmatched close-bracket at pattern character 67. (food)
2Lua error: Unmatched close-bracket at pattern character 67.
1Lua error: Unmatched close-bracket at pattern character 67.
0.25Lua error: Unmatched close-bracket at pattern character 67.

0.859367007375Lua error: Unmatched close-bracket at pattern character 67.
0.214841751844Lua error: Unmatched close-bracket at pattern character 67.

946.352946Lua error: Unmatched close-bracket at pattern character 67.
0.946352946Lua error: Unmatched close-bracket at pattern character 67.

US dry quart
(qt)

38.7575588768Lua error: Unmatched close-bracket at pattern character 67.
0.96893897192Lua error: Unmatched close-bracket at pattern character 67.
0.24223474298Lua error: Unmatched close-bracket at pattern character 67.

37.2367099567Lua error: Unmatched close-bracket at pattern character 67.
36.7073647572Lua error: Unmatched close-bracket at pattern character 67. (food)
1.16364718615Lua error: Unmatched close-bracket at pattern character 67.
0.29091179654Lua error: Unmatched close-bracket at pattern character 67.

1 US dry qt
≡ 2 US dry pt
≡ 0.25 US dry gal

1101.220942715Lua error: Unmatched close-bracket at pattern character 67.
1.101220942715Lua error: Unmatched close-bracket at pattern character 67.


gallons
Imperial gallon
(gal)

160Lua error: Unmatched close-bracket at pattern character 67.
4Lua error: Unmatched close-bracket at pattern character 67.
1Lua error: Unmatched close-bracket at pattern character 67.

153.721590465Lua error: Unmatched close-bracket at pattern character 67.
151.536333333Lua error: Unmatched close-bracket at pattern character 67. (food)
1.20094992550Lua error: Unmatched close-bracket at pattern character 67.

4.12822697395Lua error: Unmatched close-bracket at pattern character 67.
1.03205674349Lua error: Unmatched close-bracket at pattern character 67.

4546.09Lua error: Unmatched close-bracket at pattern character 67.
4.54609Lua error: Unmatched close-bracket at pattern character 67.

US liquid gallon
(gal)

133.227869541Lua error: Unmatched close-bracket at pattern character 67.
3.33069673852Lua error: Unmatched close-bracket at pattern character 67.
0.83267418463Lua error: Unmatched close-bracket at pattern character 67.

128Lua error: Unmatched close-bracket at pattern character 67.
126.1803928Lua error: Unmatched close-bracket at pattern character 67. (food)
4Lua error: Unmatched close-bracket at pattern character 67.
1Lua error: Unmatched close-bracket at pattern character 67.
231 cu inches

3.437468029501Lua error: Unmatched close-bracket at pattern character 67.
0.859367007375Lua error: Unmatched close-bracket at pattern character 67.

3785.411784Lua error: Unmatched close-bracket at pattern character 67.
3.785411784Lua error: Unmatched close-bracket at pattern character 67.

US dry gallon
(gal)

155.030235507Lua error: Unmatched close-bracket at pattern character 67.
3.87575588768Lua error: Unmatched close-bracket at pattern character 67.
0.96893897192Lua error: Unmatched close-bracket at pattern character 67.

148.946839827Lua error: Unmatched close-bracket at pattern character 67.
146.829459029Lua error: Unmatched close-bracket at pattern character 67. (food)
4.6545887446Lua error: Unmatched close-bracket at pattern character 67.
1.16364718615Lua error: Unmatched close-bracket at pattern character 67.

4 US dry qt
1 US dry gal
268.8025 cubic inches

4404.88377086Lua error: Unmatched close-bracket at pattern character 67.
4.40488377086Lua error: Unmatched close-bracket at pattern character 67.


metric
litre
(l or L or dm3)

35.1950797279Lua error: Unmatched close-bracket at pattern character 67.
0.87987699320Lua error: Unmatched close-bracket at pattern character 67.
0.21996924830Lua error: Unmatched close-bracket at pattern character 67.

33.8140227018Lua error: Unmatched close-bracket at pattern character 67.
33.3333333333Lua error: Unmatched close-bracket at pattern character 67. (food)
1.05668820943Lua error: Unmatched close-bracket at pattern character 67.
0.26417205236Lua error: Unmatched close-bracket at pattern character 67.

0.90808298427Lua error: Unmatched close-bracket at pattern character 67.
0.22702074607Lua error: Unmatched close-bracket at pattern character 67.

1000Lua error: Unmatched close-bracket at pattern character 67.
1Lua error: Unmatched close-bracket at pattern character 67.

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 through the international yard and pound agreement of 1959.

The US survey foot and survey mile have been maintained as separate units for surveying purposes to avoid the accumulation of error that would follow replacing them with the international versions, particularly with State Plane Coordinate Systems. (The choice of unit for surveying purposes is based on the unit used when the overall framework or geodetic datum for the region was established, so that - for example - much of the former British empire still uses the Clarke foot for surveying.) The US survey foot is defined so that 1 metre is exactly 39.37 inches, making the international foot of 0.3048 metres two parts per million shorter. This is a difference of just over 3.2 mm or a little over one eighth of an inch per mile.

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

At one time the definition of the nautical mile was 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

File:English mass units graph.svg
Relation of English mass weights to one another

Traditiionally, both Britain and the US used three different weight systems: troy weight for precious metals, avoirdupois weight for most other purposes and apothecaries' weight for medicines. However, apothecaries' weight has now been superseded by the metric system.

One important difference is the widespread use in Britain of the stone of 14 pounds (6.35029318Lua error: Unmatched close-bracket at pattern character 67.) 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 (373.2417216Lua error: Unmatched close-bracket at pattern character 67.) on January 6, 1879, leaving only the troy ounce (31.1034768Lua error: Unmatched close-bracket at pattern character 67.) 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 has a hundredweight of eight stone or 112 lb (50.80234544Lua error: Unmatched close-bracket at pattern character 67.), whereas a US hundredweight is 100 lb (45.359237Lua error: Unmatched close-bracket at pattern character 67.). In both systems, 20 hundredweights make a ton. In the US, the terms long ton (2240Lua error: Unmatched close-bracket at pattern character 67., 1016.0469088Lua error: Unmatched close-bracket at pattern character 67.) and short ton (2000Lua error: Unmatched close-bracket at pattern character 67.; 907.18474Lua error: Unmatched close-bracket at pattern character 67.) are used to distinguish them. The term metric ton is also used to denote a tonne (1000Lua error: Unmatched close-bracket at pattern character 67., 2204.622Lua error: Unmatched close-bracket at pattern character 67.), which is about 2% less than the long ton.

See also

Notes

  1. ^ Wine gallons, however, continued to be used for tax purposes in the UK until the late 1990s.
  2. ^ The water was to be weighed in air with brass weights with the barometer standing at Script error: No such module "convert". at a temperature of Script error: No such module "convert".. In 1963 these conditions were redefined such that the water was to have a density of 0.998859Lua error: Unmatched close-bracket at pattern character 67. and to be weighed in air of density 0.001217Lua error: Unmatched close-bracket at pattern character 67. against weights of density 8.136 g/mL.
  3. ^ Originally defined as the volume of a cylinder Script error: No such module "convert". in diameter and Script error: No such module "convert". deep, the Winchester bushel was redefined in the US as 2150.42 cubic inches.
  4. ^ The gill is no longer in common use.
  5. ^ The now rarely used apothecaries' system of fluid measures further divides the fluid ounce into 8 fluid drams or 480 minims. Also in the imperial system there is a fluid scruple of 20 minims which is absent from the US customary system. Like the fluid ounce the dram and minim are about 4% smaller in the imperial system.
  6. ^ 160 imperial fluid ounces is equivalent to one imperial gallon, which is the approximate volume of 10 pounds or 160 avoirdupois ounces of water at 62 °F.