Imperial units or the Imperial system is a collection of units, first defined in the British Weights and Measures Act of 1824, later refined (until 1959) and reduced. The units were introduced in the United Kingdom and its colonies, including Commonwealth countries (most have since become officially metric, but continue to use both Metric and Imperial), but excluding the then already independent United States. Systems of imperial units are sometimes referred to as foot-pound-second, after the base units of length, mass and time.
Although most of the units are defined in more than one system, some subsidiary units were used to a much greater extent, or for different purposes, in one area rather than the other.
After the 1 July 1959 deadline, agreed upon in 1958, the U.S. and the British yard were defined identically, at 0.9144 m to match the international yard. Metric equivalents in this article usually assume this latest official definition. Before this date, the most precise measurement of the imperial Standard Yard was 0.914398416 m (Sears et al. 1928. Phil Trans A 227:281).
|Unit||Relative value to foot||Metric value||Notes|
|thou||25.4 μm||Known as a mil in the United States|
|yard||3||91.44 cm||Defined as exactly 0.9144 metres since 1956.|
|league||15,840||4828.032 m||No longer an official unit in any nation.|
|fathom||6||~1.853 m||The British Admiralty in practice used a fathom as 6 feet. This was despite it being of a nautical mile (i.e. 6.08 feet) until 1970, when the international nautical mile of exactly 1852 metres was adopted. The commonly accepted definition of a fathom was always 6 feet. The conflict was inconsequential in determining depth as Admiralty nautical charts used feet as depths below 5 fathoms on older imperial charts. Today all charts worldwide are metric, except for USA Hydrographic Office charts, which use feet for all depth ranges.|
|cable||608||~185.3 m||One tenth of a nautical mile. When in use it was approximated colloquially as 100 fathoms.|
|nautical mile||6080||~1853 m||Used to measure distances at sea. This value referred to the British nautical (Admiralty) mile of 6,080 ft; the modern international mile is slightly different.|
|Gunter's survey units (17th century onwards)|
|pole||~5.029 m||The pole is also called rod or perch.|
Until the adoption of the international definition of 1852 metres in 1970, the British nautical mile was defined as 6080 feet. It was not readily expressible in terms of any of the intermediate units, because it was derived from the circumference of the Earth (like the original metre).
|1 perch||= 1 rod × 1 rod||= square mile||= 272.25 sq ft||= ~0.00253 ha||= ~25.29 m²|
|1 rood||= 1 furlong × 1 rod||= 40 square rods||= square mile||= 10,890 sq ft||= ~0.1012 ha||= ~1012 m²|
|1 acre||= 1 furlong × 1 chain||= 160 square rods||= square mile||= 43,560 sq ft||= ~0.4047 ha||= ~4047 m²|
Although the proper term is square rod, for centuries this unit has been called a pole or perch or, more properly square pole or square perch.
|Unit||Relative value to pint||Metric value||U.S. value||Notes|
|fluid ounce (fl oz)||~28.41 ml||~0.9608 fl oz|
|gill||~142.1 ml||~4.804 fl oz|
|pint (pt)||1||~568.2 ml||~1.201 pt||Still the compulsory serving size for beer and cider in the UK.|
|quart (qt)||2||~1.136 L||~1.201 qt|
|gallon (gal)||8||~4.546 L||~1.201 gal||Exactly 4.546 09 litres.|
For a comparison to the U.S. customary system see the article on Comparison of the imperial and US customary measurement systems.
The use of the troy pound (373.241 721 6 g) was abolished in Britain on January 6, 1879, making the Avoirdupois pound the primary unit of weight.with only the troy ounce (31.103 476 8 g) and its decimal subdivisions retained. In all the systems, the fundamental unit is the pound, and all other units are defined as fractions or multiples of it.
|Unit||Relative value to pound||Metric value||U.S. value||Notes|
|ounce (oz)||~28.35 g|
|pound (lb)||1||~453.6 g||Exactly 453.592 37 grammes.|
|stone (st)||14||~6.35 kg||A person's weight is often quoted in stone and pounds in English-speaking countries, with the exception of the United States and Canada, where it is usually quoted in pounds.|
|quarter||28||~12.7 kg||A "quarter" was also commonly used to refer to a quarter of a pound in a retail context.|
|hundredweight (cwt)||112||~50.8 kg||100 lb|
|ton (t)||2240||~1016 kg||2000 lb||20 hundredweights in both systems, US hundredweight being lighter.|
The British ton (the long ton), is 2240 pounds, which is very close to a metric tonne, whereas the ton generally used in the United States is the "short ton" of 2000 pounds (907.184 74 kg). Each is divided into 20 hundredweights (cwt), the British hundredweight of 112 pounds being 12% heavier than the American hundredweight.
British law now defines each imperial unit in terms of the metric equivalent.
The Units of Measurement Regulations 1995 require that all measuring devices used in trade or retail be capable of measuring and displaying metric quantities. This has now been proved in court against the so-called "Metric Martyrs", a small group of market traders who insisted on trading in imperial units only. Contrary to the impression given by some press reports, these regulations have never placed any obstacle in the way of using imperial units alongside metric units. Almost all traders in the UK will accept requests from customers specified in imperial units, and scales which display in both unit systems are commonplace in the retail trade. Metric price signs may currently be accompanied by imperial price signs (known as supplementary indicators) provided that the imperial signs are no larger and no more prominent than the official metric ones. The EU's deadline of December 31, 2009 to enforce metric-only labels and ban any supplementary indicators (imperial measurements) on goods after the deadline has been abolished. On May 9, 2007 the European Commission agreed to allow supplementary indications alongside the statutory metric indications beyond 2009.
The United Kingdom completed its legal transition to SI units in 1995, but a few imperial units are still in official use: draught beer must be sold in pints, road-sign distances must be in yards and miles, clearance heights must be in feet and inches (although an equivalent in metres may be shown as well) and speed limits must be in miles per hour, therefore instruments in British-registered vehicles must be capable of displaying miles per hour. (Foreign vehicles, such as all post-2005 Irish vehicles, may legally have instruments displayed only in kilometres per hour.) Even though the troy pound was outlawed in Great Britain in the Weights and Measures Act of 1878, the troy ounce still may be used for the weight of precious stones and metals. The railways are also a big user of imperial units, with distances officially measured in miles and yards or miles and chains, and also feet and inches, and speeds are in miles per hour, although many modern metro and tram systems are entirely metric, and London Underground uses both metric (for distances) and imperial (for speeds). Metric is also used for the Channel Tunnel and on High Speed 1. Adjacent to Ashford International railway station and Dollands Moor International Freight Terminal, speeds are given in both metric and imperial units.
The use of SI units is mandated by law for the retail sale of food and other commodities, but most British people still use imperial units in colloquial discussion of distance (miles) and speed (miles per hour). Milk is available in both half-litre and pint containers. Most people still measure their weight in stones and pounds, and height in feet and inches—but these must be converted to metric if recorded officially, for example in medical records. Petrol is sometimes quoted as being so much per gallon, despite having been sold exclusively in litres for two decades. Likewise, fuel consumption for cars is still usually in miles per gallon, though official figures always include litres per 100 km equivalents. Fahrenheit equivalents are occasionally given after Celsius in weather forecasts.
In the 1970s the metric system and SI units were introduced in Canada to replace the imperial system. Within the government, efforts to implement the metric system were extensive; almost any agency, institution, or function provided by the government uses SI units exclusively. Imperial units were eliminated from all road signs, although both systems of measurement will still be found on privately-owned signs, such as the height warnings at the entrance of a multi-storey parking facility. In the 1980s, momentum to fully convert to the metric system stalled when the government of Brian Mulroney was elected. There was heavy opposition to metrication and as a compromise the government maintains legal definitions for and allows use of imperial units as long as metric units are shown as well. The law requires that measured products (such as fuel and meat) be priced in metric units, although an imperial price can be shown if a metric price is present. However, there tends to be leniency in regards to fruits and vegetables being priced in imperial units only. Environment Canada still offers an imperial unit option beside metric units, even though weather is typically measured and reported in metric units in the Canadian media. However, some radio stations near the United States border (such as CIMX and CIDR) primarily use imperial units to report the weather.
Imperial units are still used in ordinary conversation. Few Canadians would exclusively use SI units to describe their weight and height. Although drivers' licences in some provinces like Ontario, Quebec, and Newfoundland and Labrador use SI units, other provinces like Saskatchewan use imperial units. In livestock auction markets, cattle are sold in dollars per hundredweight (short), whereas hogs are sold in dollars per hundred kilograms. Imperial units still dominate in recipes, construction, house renovation and gardening, although often informally. Land is now surveyed and registered in metric units, although initial surveys used imperial units. For example, partitioning of farm land on the prairies in the late 19th and early 20th centuries was done in imperial units; this accounts for imperial units of distance and area retaining wide use in the prairie provinces. The size of most apartments, condominiums and houses continues to be described in square feet rather than square metres, and carpet or flooring tile is purchased by the square foot.
Imperial units also retain common use in firearms and ammunition. Imperial measures are still used in the description of cartridge types, even when the cartridge is of relatively recent invention (e.g. 0.204 Ruger, 0.17 HMR, where the calibre is expressed in decimal fractions of an inch). However, ammunition which is classified in metric already is still kept metric (e.g. 9 mm, 7.62 mm). In the manufacture of ammunition, bullet and powder weights are expressed in terms of grains for both metric and imperial cartridges.
Some imperial measurements remain in limited use in Australia, India, Malaysia and New Zealand. Real estate agents continue to use acres and square feet to describe area in conjunction with hectares and square metres. Measurements in feet and inches, especially for a person's height, are frequently met in conversation and non-governmental publications.