This article relies largely or entirely on a single source. (August 2020) |
Both the British imperial measurement system and United States customary systems of measurement derive from earlier English unit systems used prior to 1824 that were the result of a combination of the local Anglo-Saxon units inherited from Germanic tribes and Roman units.
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, almost a half-century after American independence.
Volume
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 they differ in their specific units of volume (the bushel, gallon, fluid ounce, etc.). 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 18th century, various systems of volume measurement were in use throughout the British Empire. Wine was measured with units based on the wine 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) and 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 units were replaced with a single system based on the imperial gallon. Originally defined as the volume of 10 pounds (4.54 kg) of distilled water (under certain conditions),, then redefined by the Weights and Measures Act 1985 to be exactly 4.54609 L (277.42 cu in), the imperial gallon is 1.62% smaller than the pre-1824 ale gallon.
The Winchester measure was made obsolete in the British Empire but remained in use in the US. The Winchester bushel was replaced with an imperial bushel of eight imperial gallons. The subdivisions of the bushel were maintained.
As with US dry measures, the imperial system divides the bushel into 32 quarts or 64 pints. Thus, these imperial measures are 3.2% larger than are 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. However, whereas the US gill is divided into four US fluid ounces, the imperial gill is divided into five imperial fluid ounces. Thus, while the imperial fluid ounce is 3.924% smaller than the US fluid ounce, the imperial gallon, quart, pint and gill are all 20.095% larger than their US fluid measure counterparts.
One avoirdupois ounce of water has an approximate volume of one imperial fluid ounce at 62 °F (16.67 °C): 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 their beer in 12 imperial fl oz bottles, which are 341 mL each, while American brewers package their beer in 12 US fl oz bottles, which are 355 mL each. Consequently, Canadian bottles are labelled as 11.5 fl oz in US units when imported into the United States.
Because the standard size of Canadian beer bottles predates the adoption of the metric system in Canada, the bottles 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 United States, they are labelled as 12 fl oz.
Notes:
| ||||
| Unit name | Imperial measures | US fluid measures | US dry measures | Metric measures |
|---|---|---|---|---|
fluid ounces | ||||
| Imperial fluid ounce (fl oz) | ≡ 1 imp fl oz | ≈ 0.96075994040 US fl oz | ≡ 28.4130625 mL | |
| US fluid ounce (customary) (fl oz) | ≈ 1.04084273079 imp fl oz | ≡ 1 US fl oz | ≡ 29.5735295625 mL | |
| US fluid ounce (food nutrition labelling) (fl oz) (food) | ≈ 1.05585239184 imp fl oz | ≈ 1.01442068106 US fl oz | ≡ 30 mL | |
pints | ||||
| Imperial pint (pt) | ≡ 20 imp fl oz | ≈ 19.2151988081 US fl oz | ≈ 1.03205674349 US dry pt | ≡ 568.26125 mL |
| US liquid pint (pt) | ≈ 16.6534836926 imp fl oz | ≡ 16 US fl oz | ≈ 0.859367007375 US dry pt | ≡ 473.176473 mL |
| US dry pint (pt) | ≈ 19.3787794384 imp fl oz | ≈ 18.6183549784 US fl oz | ≡ 1 US dry pt | ≡ 550.6104713575 mL |
quarts | ||||
| Imperial quart (qt) | ≡ 40 imp fl oz | ≈ 38.4303976162 US fl oz | ≈ 1.03205674349 US dry qt | ≡ 1136.5225 mL |
| US liquid quart (qt) | ≈ 33.3069673852 imp fl oz | ≡ 32 US fl oz | ≈ 0.859367007375 US dry qt | ≡ 946.352946 mL |
| US dry quart (qt) | ≈ 38.7575588768 imp fl oz | ≈ 37.2367099567 US fl oz | ≡ 1 US dry qt | ≡ 1101.220942715 mL |
gallons | ||||
| Imperial gallon (gal) | ≡ 160 imp fl oz | ≈ 153.721590465 US fl oz | ≈ 4.12822697395 US dry qt | ≡ 4546.09 mL |
| US liquid gallon (gal) | ≈ 133.227869541 imp fl oz | ≡ 128 US fl oz | ≈ 3.437468029501 US dry qt | ≡ 3785.411784 mL |
| US dry gallon (gal) | ≈ 155.030235507 imp fl oz | ≈ 148.946839827 US fl oz | ≡ 4 US dry qt | ≡ 4404.88377086 mL |
metric | ||||
| litre (l or L or dm3) | ≈ 35.1950797279 imp fl oz | ≈ 33.8140227018 US fl oz | ≈ 0.90808298427 US dry qt | ≡ 1000 mL |
Length
The international yard is defined as exactly 0.9144 metres. This definition was approved by the United States, Canada, the United Kingdom, South Africa, Australia and New Zealand through the international yard and pound agreement of 1959, and corresponds with the previous 1930s British and American definitions of 1 inch being 25.4 mm. In all systems, a yard is 36 inches.
The US survey foot and survey mile were 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; for example, much of the former British empire still uses the Clarke foot[further explanation needed] 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 exactly two parts per million shorter. This is a difference of just over 3.2 mm, or a little more than one-eighth of an inch per mile. According to the National Institute of Standards and Technology, the survey foot is obsolete as of 1 January 2023, and its use discouraged.
The main units of length (inch, foot, yard and international mile) were the same in the US, though the US rarely uses some of the intermediate units today, 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 surface area of the Clarke ellipsoid.[further explanation needed] While the US used the full value of 1853.256 metres, in the British Commonwealth, this was rounded to 6080 feet (1853.184 m). These have been replaced by the international version (which rounds the 60th part of the 45° to the nearest metre) of 1852 metres.
Weight and mass
Traditionally, both Britain and the US used three different weight systems: troy weight for precious metals, apothecaries' weight for medicines and avoirdupois weight for almost all other purposes. 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 (6.35029318 kg) for body weight; this unit is not used in the United States, although flour was sold by a barrel of 196 pounds (14 stone) until World War II.
Another difference arose when Britain abolished the troy pound (373.2417216 g) on 1 January 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 no longer widely used.
In all of these systems, the fundamental unit is the pound (lb), and all other units are defined as fractions or multiples of a pound. 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 one pound, but above that point, the tables differ.
The imperial system has a hundredweight, defined as eight stone of 14 lb each, 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. The metric ton is the name used for the tonne (1000 kg, 2204.62262 lb), which is about 1.6% less than the long ton.
The US customary system also includes the kip, equivalent to 1,000 pounds of force, which is also occasionally used as a unit of weight of 1,000 pounds (usually in engineering contexts).
See also
- Conversion of units
- History of measurement
- Systems of measurement
- Weights and measures
Notes
- Wine gallons, however, continued to be used for tax purposes in the UK until the late 1990s.
- 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.
- Originally defined as the volume of a cylinder 18+1⁄2 inches (470 mm) in diameter and 8 inches (200 mm) deep, the Winchester bushel was redefined in the US as 2150.42 cubic inches.
- The gill is no longer in common use.
- The now rarely used apothecaries' system of fluid measures further divides the fluid ounce into 8 fluid drams or 480 minims, and 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 3.924% smaller in the imperial system.
- 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.
References
- "U.S. Survey Foot". NIST. 26 July 2019.
This article relies largely or entirely on a single source Relevant discussion may be found on the talk page Please help improve this article by introducing citations to additional sources Find sources Comparison of the imperial and US customary measurement systems news newspapers books scholar JSTOR August 2020 Both the British imperial measurement system and United States customary systems of measurement derive from earlier English unit systems used prior to 1824 that were the result of a combination of the local Anglo Saxon units inherited from Germanic tribes and Roman units A baby bottle with measurements in metric imperial and US customaryA one US gallon petrol can purchased near the US Canada border showing equivalents in imperial gallons and litres 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 almost a half century after American independence VolumeVolume 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 they differ in their specific units of volume the bushel gallon fluid ounce etc 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 18th century various systems of volume measurement were in use throughout the British Empire Wine was measured with units based on the wine 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 and 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 units were replaced with a single system based on the imperial gallon Originally defined as the volume of 10 pounds 4 54 kg of distilled water under certain conditions then redefined by the Weights and Measures Act 1985 to be exactly 4 54609 L 277 42 cu in the imperial gallon is 1 62 smaller than the pre 1824 ale gallon The Winchester measure was made obsolete in the British Empire but remained in use in the US The Winchester bushel was replaced with an imperial bushel of eight imperial gallons The subdivisions of the bushel were maintained As with US dry measures the imperial system divides the bushel into 32 quarts or 64 pints Thus these imperial measures are 3 2 larger than are 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 However whereas the US gill is divided into four US fluid ounces the imperial gill is divided into five imperial fluid ounces Thus while the imperial fluid ounce is 3 924 smaller than the US fluid ounce the imperial gallon quart pint and gill are all 20 095 larger than their US fluid measure counterparts One avoirdupois ounce of water has an approximate volume of one imperial fluid ounce at 62 F 16 67 C 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 their beer in 12 imperial fl oz bottles which are 341 mL each while American brewers package their beer in 12 US fl oz bottles which are 355 mL each Consequently Canadian bottles are labelled as 11 5 fl oz in US units when imported into the United States Because the standard size of Canadian beer bottles predates the adoption of the metric system in Canada the bottles 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 United States they 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 boldUnit name Imperial measures US fluid measures US dry measures Metric measuresfluid ouncesImperial fluid ounce fl oz 1 imp fl oz 0 960759 940 40 US fl oz 0 947102 083 33 US fl oz food 28 4130625 mL 0 028413 0625 LUS fluid ounce customary fl oz 1 040842 730 79 imp fl oz 1 US fl oz 0 985784 318 75 US fl oz food 29 573529 5625 mL 0 029573 529 5625 LUS fluid ounce food nutrition labelling fl oz food 1 055852 391 84 imp fl oz 1 014420 681 06 US fl oz 1 US fl oz food 30 mL 0 03 LpintsImperial pint pt 20 imp fl oz 1 imp pt 0 5 imp qt 0 125 imp gal 19 215198 8081 US fl oz 18 942041 6667 US fl oz food 1 200949 925 50 US pt 0 150118 740 69 US gal 1 032056 743 49 US dry pt 0 129007 092 94 US dry gal 568 26125 mL 0 568261 25 LUS liquid pint pt 16 653483 6926 imp fl oz 0 832674 184 63 imp pt 0 104084 273 08 imp gal 16 US fl oz 15 7725491 US fl oz food 1 US pt 0 5 US qt 0 125 US gal 0 859367 007 375 US dry pt 0 107420 875 922 US dry gal 473 176473 mL 0 473176 473 LUS dry pint pt 19 378779 4384 imp fl oz 0 968938 971 92 imp pt 0 121117 371 49 imp gal 18 618354 9784 US fl oz 18 353682 3786 US fl oz food 1 163647 186 15 US pt 0 145455 898 27 US gal 1 US dry pt 0 5 US dry qt 0 125 US dry gal 550 610471 3575 mL 0 550610 471 3575 LquartsImperial quart qt 40 imp fl oz 1 imp qt 2 imp pt 0 25 imp gal 38 430397 6162 US fl oz 37 884083 3333 US fl oz food 1 200949 925 50 US qt 0 300237 481 38 US gal 1 032056 743 49 US dry qt 0 258014 185 87 US dry gal 1136 5225 mL 1 1365225 LUS liquid quart qt 33 306967 3852 imp fl oz 0 832674 184 63 imp qt 0 208168 546 16 imp gal 32 US fl oz 31 5450982 US fl oz food 2 US pt 1 US qt 0 25 US gal 0 859367 007 375 US dry qt 0 214841 751 844 US dry gal 946 352946 mL 0 946352 946 LUS dry quart qt 38 757558 8768 imp fl oz 0 968938 971 92 imp qt 0 242234 742 98 imp gal 37 236709 9567 US fl oz 36 707364 7572 US fl oz food 1 163647 186 15 US qt 0 290911 796 54 US gal 1 US dry qt 2 US dry pt 0 25 US dry gal 1101 220942 715 mL 1 101220 942 715 LgallonsImperial gallon gal 160 imp fl oz 4 imp qt 1 imp gal 153 721590 465 US fl oz 151 536333 333 US fl oz food 1 200949 925 50 US gal 4 128226 973 95 US dry qt 1 032056 743 49 US dry gal 4546 09 mL 4 54609 LUS liquid gallon gal 133 227869 541 imp fl oz 3 330696 738 52 imp qt 0 832674 184 63 imp gal 128 US fl oz 126 1803928 US fl oz food 4 US qt 1 US gal 231 cu in 3 437468 029 501 US dry qt 0 859367 007 375 US dry gal 3785 411784 mL 3 785411 784 LUS dry gallon gal 155 030235 507 imp fl oz 3 875755 887 68 imp qt 0 968938 971 92 imp gal 148 946839 827 US fl oz 146 829459 029 US fl oz food 4 654588 7446 US qt 1 163647 186 15 US gal 4 US dry qt 1 US dry gal 268 8025 cu in 4404 883770 86 mL 4 404883 770 86 Lmetriclitre l or L or dm3 35 195079 7279 imp fl oz 0 879876 993 20 imp qt 0 219969 248 30 imp gal 33 814022 7018 US fl oz 33 333333 3333 US fl oz food 1 056688 209 43 US qt 0 264172 052 36 US gal 0 908082 984 27 US dry qt 0 227020 746 07 US dry gal 1000 mL 1 LLengthThe international yard is defined as exactly 0 9144 metres This definition was approved by the United States Canada the United Kingdom South Africa Australia and New Zealand through the international yard and pound agreement of 1959 and corresponds with the previous 1930s British and American definitions of 1 inch being 25 4 mm In all systems a yard is 36 inches The US survey foot and survey mile were 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 for example much of the former British empire still uses the Clarke foot further explanation needed 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 exactly two parts per million shorter This is a difference of just over 3 2 mm or a little more than one eighth of an inch per mile According to the National Institute of Standards and Technology the survey foot is obsolete as of 1 January 2023 and its use discouraged The main units of length inch foot yard and international mile were the same in the US though the US rarely uses some of the intermediate units today 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 surface area of the Clarke ellipsoid further explanation needed While the US used the full value of 1853 256 metres in the British Commonwealth this was rounded to 6080 feet 1853 184 m These have been replaced by the international version which rounds the 60th part of the 45 to the nearest metre of 1852 metres Weight and massRelation of English mass weights to one another Traditionally both Britain and the US used three different weight systems troy weight for precious metals apothecaries weight for medicines and avoirdupois weight for almost all other purposes 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 350293 18 kg for body weight this unit is not used in the United States although flour was sold by a barrel of 196 pounds 14 stone until World War II Another difference arose when Britain abolished the troy pound 373 2417216 g on 1 January 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 no longer widely used In all of these systems the fundamental unit is the pound lb and all other units are defined as fractions or multiples of a pound 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 one pound but above that point the tables differ The imperial system has a hundredweight defined as eight stone of 14 lb each or 112 lb 50 802345 44 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 The metric ton is the name used for the tonne 1000 kg 2204 62262 lb which is about 1 6 less than the long ton The US customary system also includes the kip equivalent to 1 000 pounds of force which is also occasionally used as a unit of weight of 1 000 pounds usually in engineering contexts See alsoConversion of units History of measurement Systems of measurement Weights and measuresNotesWine gallons however continued to be used for tax purposes in the UK until the late 1990s 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 Originally defined as the volume of a cylinder 18 1 2 inches 470 mm in diameter and 8 inches 200 mm deep the Winchester bushel was redefined in the US as 2150 42 cubic inches The gill is no longer in common use The now rarely used apothecaries system of fluid measures further divides the fluid ounce into 8 fluid drams or 480 minims and 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 3 924 smaller in the imperial system 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 References U S Survey Foot NIST 26 July 2019
