Imperial units

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.

Relation to other systems

The distinction between the imperial system and the U.S. customary units (also called standard or English units) or older British/English units/systems and newer additions is often not drawn precisely. Most length units are shared between the imperial and U.S. systems, albeit partially and temporally defined differently. Capacity measures differ the most due to the introduction of the imperial gallon and the unification of wet and dry measures. The avoirdupois system applies only to weights; it has a long designation and a short designation for the hundredweight and ton. The term imperial should not be applied to English units that were outlawed in Weights and Measures Act of 1824 or earlier, or which had fallen out of use by that time, nor to post-imperial inventions such as the slug or poundal.

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.

Measures of length

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).

Table of length equivalent units
Unit Relative value to foot Metric value Notes
thou 25.4 μm Known as a mil in the United States
inch 2.54 cm
foot 1 30.48 cm
yard 3 91.44 cm Defined as exactly 0.9144 metres since 1956.
furlong 660 201.168 m
mile 5280 1609.344 m
league 15,840 4828.032 m No longer an official unit in any nation.
Maritime units
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)
link ~20.12 cm
pole ~5.029 m The pole is also called rod or perch.
chain 66 ~20.12 m

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).

Measures of area

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.

The rood is also called a rod - see this catalogue of old documents with many areas quoted as "acres rods perches" including this one as recent as 1907.

Measures of volume

In 1824, Britain adopted a close approximation to the ale gallon known as the imperial gallon. The imperial gallon was based on the volume of 10 lb of distilled water weighed in air with brass weights with the barometer standing at 30 in Hg at a temperature of 62 °F. In 1963 this definition was refined as the space occupied by 10 lb of distilled water of density 0.998 859 g/ml weighed in air of density 0.001 217 g/ml against weights of density 8.136 g/ml. This works out to 4.545 964 591 L, or 277.420 cu in. The Weights and Measures Act of 1985 switched to a gallon of exactly 4.546 09 L (approximately 277.4 cu in) .

Table of volume units
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.

Measures of weight and mass

Britain has used three different weight systems in the 19th and 20th centuries: troy weight, used for precious metals; avoirdupois weight, used for most other purposes; and apothecaries' weight, now virtually unused since the metric system is used for all scientific purposes. The 1824 Act made the Troy pound the primary unit of weight.

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.

Table of mass units
Unit Relative value to pound Metric value U.S. value Notes
grain ~64.8 mg
drachm ~1.771 g
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.

Current use of imperial units

United Kingdom

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.

Australia, India, Malaysia and New Zealand

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.

Republic of Ireland

The Republic of Ireland has officially changed over to the metric system since entering the European Union, with distances on new road signs being metric since 1977 and speed limits being metric since 2005. However the imperial system remains in limited use, particularly for sales of beer in pubs (traditionally sold by the pint) and some other commodities (such as milk). A minority of old road signs with distances in miles still remain, and the majority of cars sold pre-2005 feature speedometers with miles per hour. The imperial system is still often used in everyday conversation, particularly by the elder generation.

United States

Imperial units are not used in the United States. The customary units in use there are historically derived from units which were in use in England at the time of settlement. The measurements of most of these units in England itself were subsequently changed.

Other countries

Petrol/gasoline is still sold by the imperial gallon in Antigua and Barbuda, Belize, Burma, Grenada, Guyana, Sierra Leone and the United Arab Emirates.

See also


External links

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