Comparison of the imperial and US customary measurement systems

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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[edit]

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.

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 10 pounds (4.5 kg) of distilled water (under certain conditions),[note 2] then redefined by The Weights and Measures Act of 1985 to be exactly 4.54609 L (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)

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

1,136.5225 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

1,101.220942715 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

4,546.09 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

3,785.411784 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

1,000 mL
1 L

Length[edit]

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.

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[edit]

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.

Relation of English mass weights to each other


See also[edit]

Notes[edit]

  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 30 inches of mercury (102 kPa) at a temperature of 62 °F (17 °C). In 1963 these conditions were redefined such that the water was to have a density of 0.998859 g/mL and to be weighed in air of density 0.001217 g/mL against weights of density 8.136 g/mL.
  3. ^ Originally defined as the volume of a cylinder 18 12 inches (470 mm) in diameter and 8 inches (200 mm) 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.