Comparison Of The Imperial And US Customary Measurement Systems
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Both the British imperial measurement system and United States customary
systems of measurement A system of units of measurement, also known as a system of units or system of measurement, is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defi ...
derive from earlier English unit systems used prior to 1824 that were the result of a combination of the local
Anglo-Saxon The Anglo-Saxons, in some contexts simply called Saxons or the English, were a Cultural identity, cultural group who spoke Old English and inhabited much of what is now England and south-eastern Scotland in the Early Middle Ages. They traced t ...
units inherited from
Germanic tribes The Germanic peoples were tribal groups who lived in Northern Europe in Classical antiquity and the Early Middle Ages. In modern scholarship, they typically include not only the Roman-era ''Germani'' who lived in both ''Germania'' and parts ...
and
Roman units The units of measurement of ancient Rome were generally consistent and well documented. Length The basic unit of Roman linear measurement was the ''pes'' (plural: ''pedes'') or Roman foot. Investigation of its relation to the English units#Leng ...
. 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 The gallon is a unit of volume in British imperial units and United States customary units. The imperial gallon (imp gal) is defined as , and is or was used in the United Kingdom and its former colonies, including Ireland, Canada, Australia ...
, 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 units based on the wine gallon of 231 cubic inches (3.785 L), while 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 of distilled water (under certain conditions), then redefined by the Weights and Measures Act 1985 to be exactly (≈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 until the 1990s. 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 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 counterparts. One avoirdupois ounce of water has an approximate volume of one imperial fluid ounce at 62 °F (16.7 °C): 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 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 (12.5 imperial fl oz), and when exported to the United States, they are labelled as 12 fl oz.


Length

The international
yard The yard (symbol: yd) is an English units, English unit of length in both the British imperial units, imperial and US United States customary units, customary systems of measurement equalling 3 foot (unit), feet or 36 inches. Sinc ...
is defined as exactly 0.9144
metre The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of ...
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 The foot (: feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is an organ at the terminal part of the leg made up o ...
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 Surveying or land surveying is the technique, profession, art, and science of determining the land, terrestrial Plane (mathematics), two-dimensional or Three-dimensional space#In Euclidean geometry, three-dimensional positions of Point (geom ...
purposes is based on the unit used when the overall framework or
geodetic datum A geodetic datum or geodetic system (also: geodetic reference datum, geodetic reference system, or geodetic reference frame, or terrestrial reference frame) is a global datum reference or reference frame for unambiguously representing the positi ...
for the region was established; 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 
inch The inch (symbol: in or prime (symbol), ) is a Units of measurement, unit of length in the imperial units, British Imperial and the United States customary units, United States customary System of measurement, systems of measurement. It is eq ...
es, 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 National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into Outline of p ...
, 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 The mile, sometimes the international mile or statute mile to distinguish it from other miles, is a imperial unit, British imperial unit and United States customary unit of length; both are based on the older English unit of Unit of length, le ...
) 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 A furlong is a measure of distance in imperial units and United States customary units equal to one-eighth of a mile, equivalent to any of 660 foot (unit), feet, 220 yards, 40 rod (unit), rods, 10 chain (unit), chains, or a ...
(220 yards). At one time, the definition of the
nautical mile A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. Historically, it was defined as the meridian arc length corresponding to one minute ( of a degree) of latitude at t ...
was based on the surface area of the Clarke ellipsoid. 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 Troy weight is a system of units of mass that originated in the Kingdom of England in the 15th century and is primarily used in the precious metals industry. The troy weight units are the grain, the pennyweight (24 grains), the troy ounce (20 ...
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 (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 In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its Chemical compound, chemical composition, and the way in which it is formed. Rocks ...
of 14 pounds () for body weight, whereas this unit is not used in the United States, although
flour Flour is a powder made by Mill (grinding), grinding raw grains, List of root vegetables, roots, beans, Nut (fruit), nuts, or seeds. Flours are used to make many different foods. Cereal flour, particularly wheat flour, is the main ingredie ...
was sold by a barrel of 196 pounds (14 stone) until World War II. Another difference arises in that Britain deleted the
troy pound Troy weight is a system of Physical unit, units of mass that originated in the Kingdom of England in the 15th century and is primarily used in the precious metals industry. The troy weight units are the Grain (unit), grain, the pennyweight (24 ...
() and the
pennyweight A pennyweight (dwt) is a unit of mass equal to 24 grains, of a troy ounce, of a troy pound, avoirdupois ounce and exactly 1.55517384 grams. It is abbreviated dwt, ''d'' standing for ''denarius'' (an ancient Roman coin), and later ...
from the relevant statute on 1 January 1879, leaving only the troy ounce () and its
decimal The decimal numeral system (also called the base-ten positional numeral system and denary or decanary) is the standard system for denoting integer and non-integer numbers. It is the extension to non-integer numbers (''decimal fractions'') of th ...
subdivisions, whereas the troy pound (of 12 troy ounces) and
pennyweight A pennyweight (dwt) is a unit of mass equal to 24 grains, of a troy ounce, of a troy pound, avoirdupois ounce and exactly 1.55517384 grams. It is abbreviated dwt, ''d'' standing for ''denarius'' (an ancient Roman coin), and later ...
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 lb each, or 112 lb (), whereas a US hundredweight is 100 lb (): in both systems, 20 hundredweights make a ton. In the US, the terms long ton (, ) and short ton (, ) are used. The metric ton is the name used for the
tonne The tonne ( or ; symbol: t) is a unit of mass equal to 1,000  kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton in the United States to distinguish it from the non-metric units of the s ...
(, ), 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, 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 A system of units of measurement, also known as a system of units or system of measurement, is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defi ...
* Weights and measures


Notes


References

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