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This article is about the post-1824 measures used in the British Empire and countries in the British sphere of influence. For information about the units used in England before 1824, see English units. For information about the units used in the US, see United States customary units. For information about the system of weight, see Avoirdupois.

Imperial units or the imperial system is a system of units, first defined in the British Weights and Measures Act of 1824, later refined (until 1959) and reduced. The system came into official use across the British Empire. By the late 20th century all nations of the former empire had officially adopted the metric system as their main system of measurement.

Relation to other systems

The imperial system is one of many systems of English or foot-pound-second units, so named because of 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. The distinctions between these systems are often not drawn precisely.

One such system is the US customary system, which is historically derived from units which were in use in England at the time of settlement. Because the United States was already independent at the time, these units were unaffected by the introduction of the imperial system. Units of length and area are mostly shared between the imperial and US systems, albeit being 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.

Another distinction to be noted is that between these systems and older British/English units/systems or newer additions. 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.



Since 1959, the US and the British yard have been defined identically to be 0.9144 metres, 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 metres.

Table of length equivalent units
Unit Relative to previous Feet Millimetres Metres Notes
thou 0.0254 25.4 μm
inch 1000 thou 25.4
foot 12 inches 1 304.8 0.3048
yard 3 feet 3 914.4 0.9144 Defined as exactly 0.9144 metres since 1959
furlong 220 yards 660 201.168
mile 8 furlongs 5,280 1,609.344
league 3 miles 15,840 4,828.032 No longer an official unit in any nation.
Maritime units
fathom 6.08 or 6 1,853.184 The British Admiralty in practice used a fathom as 6 feet. This was despite its being of a nautical mile (i.e. 6.08 feet) until the adoption of the international nautical mile. 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 ~100 fathoms 608 185.3184 One tenth of a nautical mile. When in use it was approximated colloquially as 100 fathoms.
nautical mile 10 cables 6,080 1,853.184 Used to measure distances at sea. Until the adoption of the international definition of 1852 metres in 1970, the British nautical (Admiralty) mile was defined as 6,080 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).
Gunter's survey units (17th century onwards)
link 201.168
pole 25 links 5,029.2 5.0292 The pole is also called rod or perch.
chain 4 poles 66 20.1168 furlong


Unit Relation to units of length Square feet Square rods Square miles Square metres Hectares Notes
perch 1 rod × 1 rod 272.25 1 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.
rood 1 furlong × 1 rod 10,890 40 0.1012 The rood is also called a rod.
acre 43,560 160   0.4047
Note: All equivalences are exact except the hectares, which are accurate to four significant figures.


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 Imperial ounce Imperial pint Millilitres Cubic inches US ounces US pints
fluid ounce (fl oz) 1
gill 5
pint (pt) 20 1
quart (qt) 40 2
gallon (gal) 160 8
Note: The millilitre equivalences are exact whereas the conversions to cubic-inch and US measures are correct to five significant figures.

For a comparison to the US customary system see the article on Comparison of the imperial and US customary measurement systems.


In the 19th and 20th centuries Britain has used three different systems for mass and weight:

The troy pound ( ) was made the primary unit of mass by the 1824 Act; however, its use was abolished in Britain on 6 January 1879, making the Avoirdupois pound the primary unit of mass with only the troy ounce ( ) 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 Pounds Grams Kilograms Notes
grain Exactly milligrams.
drachm 1.7718451953125
ounce (oz)
pound (lb) 1 Exactly grams by definition.
stone (st) 14 A person's weight is often quoted in stone and pounds in English-speaking countries using the imperial system, with the exception of the United States and Canada, where it is usually quoted in pounds.
quarter 28 A "quarter" was also commonly used to refer to a quarter of a pound in a retail context.
hundredweight (cwt) 112
ton (t) 2240 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 (907.18474 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

A baby bottle that measures in three measurement systems—metric, imperial (UK), and US customary.

United States

The United States has never utilized the imperial system. The established method of measurement in the United States is the United States customary system, which differs from those of the imperial.

United Kingdom

British law now defines each imperial unit in terms of the metric equivalent. The metric system is in official use within the United Kingdom; however, use of Imperial unit is widespread in many cases.

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 proven 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 31 December 2009 to enforce metric-only labels and ban any supplementary indicators (imperial measurements) on goods after the deadline has been abolished. On 9 May 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 many imperial units are still in official use: draught beer must be sold in pints, road-sign distances must be in yards and miles, road-sign length or width (but not weight) restrictions must be in feet and inches (although an equivalent in metres may be shown as well) and road speed limits must be in miles per hour, therefore instruments in vehicles sold in Britain 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 chain, 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 Tunnelmarker and on High Speed 1. Adjacent to Ashford International railway stationmarker and Dollands Moor Freight Yardmarker, railway 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 stone 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. Threads on non metric nuts and bolts etc are sometimes referred to as Imperial, especially in the UK.


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 CIMXmarker and CIDRmarker) primarily use imperial units to report the weather.

Imperial units are still used in ordinary conversation. Few older Canadians would exclusively use SI units to describe their weight and height; newborns are measured in SI at hospitals, but the birth weight and length is usually announced to family and friends in imperial units. Although drivers' licences in some provinces like British Columbia, Alberta, Manitoba, 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. Motor-vehicle fuel consumption is reported in both litres per 100 km and statute miles per imperial gallon, leading to the erroneous impression that Canadian vehicles are 20% more fuel-efficient than their apparently-identical American counterparts, their fuel economy being reported in statute miles per US gallon. (neither country specifies which gallon is used)

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.

As in most of the western world, air navigation is based on nautical units, e.g. the nautical mile, which is neither imperial nor metric.

Other Countries

Some imperial measurements remain in limited use in India, Malaysia, South Africa and Hong Kong. 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.

Towns and villages in Malaysia with no proper names had adopted the mile (known also as batu in Malay) to denote their locations along a rural main road (i.e. "3rd Mile", "Batu Enam" or "Batu 11"); many of their names remain unchanged even after the adoption of the metric system for distance in the country.

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. The imperial system remains in limited use - for sales of beer in pubs (traditionally sold by the pint). All other goods are required by law to be sold in metric units, although old quantities are retained for some goods like butter, which is sold in 454-gram (1 lb) packaging. 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 older generation.

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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