View source for Comparison of the imperial and US customary measurement systems

{{Use dmy dates|date=July 2023}}
{{Use British English|date=July 2023}}
{{short description|none}}
{{outline|Imperial and US customary measurement systems}}
{{one source|date=August 2020}}
[[Image:MetricImperialUSCustomaryUnits.jpg|200px|thumb|right|A baby bottle with measurements in metric, imperial and US customary]]
[[Image:GasCan.jpg|200px|thumb|right|A one-US-gallon petrol can purchased near the US–Canada border showing equivalents in imperial gallons and litres]]

Both the [[British imperial measurement system]] and [[United States customary units|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-Saxons|Anglo-Saxon]] units inherited from [[Germanic tribes]] and [[Ancient Roman weights and measures|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 [[Declaration of Independence (United States)|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 a set of units for fluids, but a different measure for dry goods 
for the pint, quart and barrel (alongside the dry goods only peck and bushel). The imperial system has only one set of measurements, defined independently of the US customary system.

By the end of the 18th century, various systems of volume measurement were in use throughout the British Empire. Wine was measured with [[English wine cask units|units]] based on the [[wine gallon]] of 231 cubic inches (3.785&nbsp;L), while beer was measured with [[English brewery cask units|units]] based on an ale gallon of 282 cubic inches (4.621&nbsp;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&nbsp;L). In 1824, these units were replaced with a single system based on the imperial gallon.{{efn|Wine gallons, however, continued to be used for tax purposes in the UK until the late 1990s.}} Originally defined as the volume of {{convert|10|lb|kg|2|lk=in}} of distilled water (under certain conditions){{efn|The water was to be weighed in air with brass weights with the barometer standing at {{convert|30|inHg|0|lk=in}} at a temperature of {{convert|62|°F|°C|0|lk=in}}. In 1963 these conditions were redefined such that the water was to have a density of {{val|0.998859|u=g/mL}} and to be weighed in air of density {{val|0.001217|u=g/mL}} against weights of density 8.136&nbsp;g/mL.}}, then redefined by the [[Weights and Measures Act 1985]] to be exactly {{val|4.54609|u=L}} (≈277.42&nbsp;cu&nbsp;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 until the 1990s.{{efn|Originally defined as the volume of a cylinder {{convert|18+1/2|in}} in diameter and {{convert|8|in}} deep, the Winchester bushel was redefined in the US as {{val|2150.42}} cubic inches.}} In the British Empire, the Winchester bushel was replaced with an imperial bushel of eight imperial gallons, with the subdivisions of the bushel being maintained. 

As with US dry measures, the imperial system divides the bushel into 4 pecks, 32 quarts or 64 pints: the imperial quart and imperial pint are 3.21% larger than their US dry counterparts, whereas the imperial peck and imperial bushel were deleted from the relevant UK statute in 1968.

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 gallon, being legally adopted in 1836. Both the imperial and US fluid gallon are divided into 4 quarts, 8 pints or 32 [[Gill (unit)|gills]].{{efn|The gill is no longer in common use.}} 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 counterparts.{{efn|The now rarely used [[apothecaries' system]] of fluid measures further divides the fluid ounce into 8 [[dram (unit)|fluid drams]] or 480 [[minim (unit)|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.}} 

One [[avoirdupois|avoirdupois ounce]] of water has an approximate volume of one imperial fluid ounce at 62&nbsp;°F (16.7&nbsp;°C){{efn|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&nbsp;°F.}}: this convenient fluid ounce to avoirdupois ounce relation does not exist in the US system, as one US fluid ounce is 4.318% larger than the avoirdupois ounce.

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&nbsp;fl&nbsp;oz bottles, which are 341&nbsp;mL each, while American brewers package their beer in 12 US&nbsp;fl&nbsp;oz bottles, which are 355&nbsp;mL each. Consequently, Canadian bottles are labelled as 11.5&nbsp;fl&nbsp;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&nbsp;mL. Canned beer in Canada is sold and labelled in 355&nbsp;mL cans (12.5 imperial&nbsp;fl&nbsp;oz), and when exported to the United States, they are labelled as 12&nbsp;fl&nbsp;oz.

{| class="wikitable"
|+ Comparison of current imperial, US and metric volume measures{{anchor|Table}}
|colspan=5 style="background:white"|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
|------------
|-
!colspan=5 style="background:white"|<br/>fluid ounces
|------------ style="background:#FFDDDD"
! Imperial fluid ounce<br />(fl oz)
|
≡ '''{{val|1|u=imp fl oz}}'''
|
≈    {{val|0.9607599|u=US fl oz}}<br />
≡    {{val|0.94710208333|u=US fl oz}} (food)
|
|
≡    {{val|28.4130625|u=mL}}<br />
≡    {{val|0.0284130625|u=L}}
|------------ style="background:#DDDDFF"
! US fluid ounce<br />(customary)<br />(fl oz)
|
≈    {{val|1.0408427|u=imp fl oz}}
|
≡ '''{{val|1|u=US fl oz}}'''<br />
≡    {{val|0.98578431875|u=US fl oz}} (food)
|
|
≡    {{val|29.5735295625|u=mL}}<br />
≡    {{val|0.0295735295625|u=L}}
|------------ style="background:#FFFFDD"
! US fluid ounce<br />(food nutrition labelling)<br />(fl oz) (food)
|
≈    {{val|1.0558524|u=imp fl oz}}
|
≈    {{val|1.0144207|u=US fl oz}}<br />
≡ '''{{val|1|u=US fl oz}} (food)'''
|
|
≡ '''{{val|30|u=mL}}'''<br />
≡    {{val|0.03|u=L}}
|-
!colspan=5 style="background:white"|<br/>pints
|------------  style="background:#FFDDDD"
! Imperial pint <br />(pt)
|   
≡ '''{{val|20|u=imp fl oz}}'''<br />
≡ '''{{val|1|u=imppt}}'''<br />
≡    {{val|0.5|u=impqt}}<br />
≡    {{val|0.125|u=impgal}}
|
≈    {{val|19.2152|u=US fl oz}}<br />
≈    {{val|18.9420416667|u=US fl oz}} (food)<br />
≈    {{val|1.20095|u=USpt}}<br />
≈    {{val|0.15011874|u=USgal}}
|
≈    {{val|1.0320567|u=USdrypt}}
|
≡    {{val|568.26125|u=mL}}<br />
≡    {{val|0.56826125|u=L}}
|------------ style="background:#DDDDFF"
! US liquid pint<br />(pt)
|
≈    {{val|16.6534836|u=impfloz}}<br />
≈    {{val|0.8326742|u=imppt}}<br />
≈    {{val|0.1040843|u=impgal}}
|
≡ '''{{val|16|u=USfloz}}'''<br />
≡    {{val|15.7725491|u=USfloz}} (food)<br />
≡ '''{{val|1|u=USpt}}'''<br />
≡    {{val|0.5|u=USqt}}<br />
≡    {{val|0.125|u=USgal}}
|
≈    {{val|0.859367|u=USdrypt}}<br />
|
≡    {{val|473.176473|u=mL}}<br />
≡    {{val|0.473176473|u=L}}
|------------ style="background:#DDFFDD"
! US dry pint<br/>(pt)
|   
≈    {{val|19.3787794|u=impfloz}}<br />
≈    {{val|0.968939|u=imppt}}<br />
≈    {{val|0.1211174|u=impgal}}
|
≈    {{val|18.618355|u=USfloz}}<br />
≈    {{val|18.353682|u=USfloz}} (food)<br />
≈    {{val|1.1636472|u=USpt}}<br />
≈    {{val|0.1454559|u=USgal}}
|
≡ '''1 US dry pt'''<br />
≡    0.5 US dry qt<br />
|
≡    {{val|550.6104713575|u=mL}}<br />
≡    {{val|0.5506104713575|u=L}}
|-
!colspan=5 style="background:white"|<br/>quarts
|------------ style="background:#FFDDDD"
! Imperial quart<br />(qt)
|   
≡ '''{{val|40|u=impfloz}}'''<br />
≡ '''{{val|1|u=impqt}}'''<br />
≡    {{val|2|u=imppt}}<br />
≡    {{val|0.25|u=impgal}}
|
≈    {{val|38.4304|u=USfloz}}<br />
≈    {{val|37.8840833333|u=USfloz}} (food)<br />
≈    {{val|1.200945|u=USqt}}<br />
≈    {{val|0.3002375|u=USgal}}
|
≈    {{val|1.0320567|u=USdryqt}}
|
≡    {{val|1136.5225|u=mL}}<br />
≡    {{val|1.1365225|u=L}}
|------------ style="background:#DDDDFF"
! US liquid quart<br />(qt)
|   
≈    {{val|33.3069674|u=impfloz}}<br />
≈    {{val|0.8326742|u=impqt}}<br />
≈    {{val|0.2081685|u=impgal}}
|
≡ '''{{val|32|u=USfloz}}'''<br />
≡    {{val|31.5450982|u=USfloz}} (food)<br />
≡    {{val|2|u=USpt}}<br />
≡ '''{{val|1|u=USqt}}'''<br />
≡    {{val|0.25|u=USgal}}
|
≈    {{val|0.859367|u=USdryqt}}<br />
|
≡    {{val|946.352946|u=mL}}<br />
≡    {{val|0.946352946|u=L}}
|------------ style="background:#DDFFDD"
! US dry quart<br/>(qt)
|   
≈    {{val|38.7575589|u=impfloz}}<br />
≈    {{val|0.968939|u=impqt}}<br />
≈    {{val|0.2422347|u=impgal}}
|
≈    {{val|37.23671|u=USfloz}}<br />
≈    {{val|36.7073648|u=USfloz}} (food)<br />
≈    {{val|1.1636472|u=USqt}}<br />
≈    {{val|0.2909118|u=USgal}}
|
≡ '''1 US dry qt'''<br />
≡    2 US dry pt<br />
|
≡    {{val|1101.220942715|u=mL}}<br />
≡    {{val|1.101220942715|u=L}}
|-
!colspan=5 style="background:white"|<br/>gallons
|------------ style="background:#FFDDDD"
! Imperial gallon<br />(gal)
|
≡    {{val|160|u=impfloz}}<br />
≡ '''{{val|4|u=impqt}}'''<br />
≡ '''{{val|1|u=impgal}}'''
|
≈    {{val|153.7216|u=USfloz}}<br />
≈    {{val|151.536333333|u=USfloz}} (food)<br />
≈    {{val|1.20095|u=USgal}}
|
≈    {{val|4.128227|u=USdryqt}}<br />
|
≡    {{val|4546.09|u=mL}}<br />
≡ '''{{val|4.54609|u=L}}'''
|------------ style="background:#DDDDFF"
! US gallon<br />(gal)
|
≈    {{val|133.2278695|u=impfloz}}<br />
≈    {{val|3.3306967|u=impqt}}<br />
≈    {{val|0.8326742|u=impgal}}
|
≡    {{val|128|u=USfloz}}<br />
≡    {{val|126.1803928|u=USfloz}} (food)<br />
≡ '''{{val|4|u=USqt}}'''<br />
≡ '''{{val|1|u=USgal}}'''<br />
≡ '''231 cu in'''
|
≈    {{val|3.437468|u=USdryqt}}<br />
|
≡    {{val|3785.411784|u=mL}}<br />
≡    {{val|3.785411784|u=L}}
|-
!colspan=5 style="background:white"|<br/>metric
|------------ style="background:#EEEEEE"
! litre<br />(l or L or dm<sup>3</sup>)
|
≈    {{val|35.1950797|u=impfloz}}<br />
≈    {{val|0.879877|u=impqt}}<br />
≈    {{val|0.21996925|u=impgal}}
|
≈    {{val|33.8140227|u=USfloz}}<br />
≈    {{val|33.3333333333|u=USfloz}} (food)<br />
≈    {{val|1.0566882|u=USqt}}<br />
≈    {{val|0.26417205|u=USgal}}
|
≈    {{val|0.908083|u=USdryqt}}<br />
|
≡ '''{{val|1000|u=mL}}'''<br />
≡ '''{{val|1|u=L}}'''
|}

==Length==
The international [[yard]] is defined as exactly 0.9144 [[metre]]s. 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 (unit)|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 System]]s. The choice of unit for [[surveying]] purposes is based on the unit used when the [[Earth ellipsoid|overall framework]] or [[geodetic datum]] for the region was established; for example, much of the former British empire still uses the Clarke foot{{Explain|date=November 2021}} for surveying.

The US survey foot is defined so that 1 metre is exactly 39.37&nbsp;[[inch]]es, making the international foot of 0.3048&nbsp;metres exactly two parts per million shorter. This is a difference of just over 3.2&nbsp;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.<ref>{{cite journal | url=https://www.nist.gov/pml/us-surveyfoot | title=U.S. Survey Foot | journal=NIST | date=26 July 2019 }}</ref>

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 [[chain (unit)|(surveyor's) chain]] (22&nbsp;yards) and the [[furlong]] (220&nbsp;yards).

At one time, the definition of the [[nautical mile]] was based on the surface area of the [[Alexander Ross Clarke|Clarke ellipsoid]].{{Explain|date=November 2021}} While the US used the full value of 1853.256 metres, in the British Commonwealth, this was rounded to 6080&nbsp;feet (1853.184&nbsp;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==
[[File:English mass units graph.svg|250px|thumbnail|right|Relation 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.

In all of these systems, the fundamental unit is the [[pound (mass)|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.

One important difference is the widespread use in Britain of the [[stone (weight)|stone]] of 14 pounds ({{val|6.35029318|u=kg}}) for body weight, whereas 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 arises in that Britain deleted the [[troy pound]] ({{val|373.2417216|u=g}}) and the [[pennyweight]] from the relevant statute on 1 January 1879, leaving only the [[troy ounce]] ({{val|31.1034768|u=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.

The imperial system has a hundredweight defined as eight stone of 14&nbsp;lb each, or 112&nbsp;lb ({{val|50.80234544|u=kg}}), whereas a US hundredweight is 100&nbsp;lb ({{val|45.359237|u=kg}}): in both systems, 20 hundredweights make a [[ton]].

In the US, the terms [[long ton]] ({{val|2240|u=lb}}, {{val|1016.0469088|u=kg}}) and [[short ton]] ({{val|2000|u=lb}},  {{val|907.18474|u=kg}}) are used. The metric ton is the name used for the [[tonne]] ({{val|1000|u=kg}}, {{val|2204.62262|u=lb}}), which is 1.58% less than the long ton and is 10.23% more than the short ton.

The [[US customary system]] also includes the [[kip (unit)|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==
{{notelist}}

==References==
{{reflist}}

{{Imperial units}}
{{United States Customary Units}}
{{systems of measurement}}

{{DEFAULTSORT:Comparison Of The Imperial And US Customary Measurement Systems}}
[[Category:Systems of units]]
[[Category:Imperial units]]
[[Category:Customary units of measurement in the United States]]
[[Category:Scientific comparisons]]