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Imperial unit

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Imperial unit

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

The system of imperial units or the imperial system (also known as British Imperial[1]) is the system of units first defined in the British Weights and Measures Act of 1824, which was later refined and reduced. The system came into official use across the British Empire. By the late 20th century, most nations of the former empire had officially adopted the metric system as their main system of measurement, but some Imperial units are still used in the United Kingdom and Canada.


The Weights and Measures of Act of 1824 was initially scheduled to go into effect on 1 May 1825.[2] However, the Weights and Measures Act of 1825 pushed back the date to 1 January 1826.[3] The 1824 Act allowed the continued use of pre-imperial units provided that they were customary, widely known, and clearly marked with imperial equivalents.[2]

Apothecaries' units

Apothecaries' units are mentioned neither in the act of 1824 nor 1825. At the time, apothecaries' weights and measures were regulated "in England, Wales, and Berwick-upon-Tweed" by the London College of Physicians, and in Ireland by the Dublin College of Physicians. In Scotland, apothecaries' units were unofficially regulated by the Edinburgh College of Physicians. The three colleges published, at infrequent intervals, pharmacopoeiae, the London and Dublin editions having the force of law.[4][5]

Imperial apothecaries' measures, based on the imperial pint of 20 fluid ounces, were introduced by the publication of the London Pharmacopoeia of 1836,[6][7] the Edinburgh Pharmacopoeia of 1839,[8] and the Dublin Pharmacopoeia of 1850.[9] The Medical Act of 1858 transferred to the Crown the right to publish the official pharmacopoeia and to regulate apothecaries' weights and measures.[10]



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 metres.[11]

Table of length equivalent units
Unit Relative to previous Feet Millimetres Metres Notes
thou (th) 1/12000 0.0254 0.0000254
Also 25.4 μm
inch (in) 1000 thou 1/12 25.4 0.0254
foot (ft) 12 inches 1 304.8 0.3048
yard (yd) 3 feet 3 914.4 0.9144
Defined as exactly 0.9144 metre by the International yard and pound agreement of 1959
chain (ch) 22 yards 66 20116.8 20.1168
The distance between the two wickets on a cricket pitch
furlong (fur) 10 chains 660 201.168
220 yards
mile (mi) 8 furlongs 5,280 1,609.344
1760 yards
league (lea) 3 miles 15,840 4,828.032
No longer an official unit in any nation.
Maritime units
fathom (ftm) ~2 yards 6.08 or 6[12] 1,853.184 1.853184
The British Admiralty in practice used a fathom as 6 feet. This was despite its being 1/1000 of a nautical mile (i.e. 6.08 feet) until the adoption of the international nautical mile.[12]
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 for measuring distances at sea. Until the adoption of the international definition of 1,852 metres in 1970, the British nautical (Admiralty) mile was defined as 6,080 feet.[13]
Gunter's survey units (17th century onwards)
link 7.92 inches 66/100 201.168 0.201168
1/100 of a chain
rod 25 links 66/4 5,029.2 5.0292
The rod is also called pole or perch and equal to 5.5 yards
chain 4 rods 66 20.1168
100 links or 1/10 of a furlong


Unit Relation to
units of length
Square feet Square rods Square miles Square metres Hectares Notes
perch 1 rod 272.25 1 1/102400 25.29285264   0.002529
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 rod[14] 10,890 40 1/2560 1011.7141056 0.1012
The rood is 1,210 square yards.
acre 1 furlong × 1 chain 43,560 160 1/640  4046.8564224 0.4047
One acre is 4,840 square yards
Note: All equivalences are exact except hectares, which are accurate to 4 significant figures.


In 1824, the various different gallons in use in the British Empire were replaced by the imperial gallon, a unit close in volume to the ale gallon. It was originally defined as the volume of 10 pounds (4.5 kg) of distilled water weighed in air with brass weights with the barometer standing at 30 inches of mercury (102 kPa) at a temperature of 62 °F (17 °C). In 1963, the gallon was redefined as the volume of 10 pounds of distilled water of density 0.998859 g/mL weighed in air of density 0.001217 g/mL against weights of density 8.136 g/mL, which works out to 4.546096 L or 277.4198 cu in. The Weights and Measures Act of 1985 switched to a gallon of exactly 4.54609 L (approximately 277.4194 cu in).[15]

Table of volume units
Unit Imperial
Millilitres Cubic inches US ounces US pints
fluid ounce (fl oz) 1     1/20     28.4130625 1.7339 0.96076 0.060047
gill (gi) 5     1/4     142.0653125 8.6694 4.8038 0.30024
pint (pt) 20     1     568.26125 34.677 19.215 1.2009
quart (qt) 40     2     1,136.5225 69.355 38.430 2.4019
gallon (gal) 160     8     4,546.09 277.42 153.72 9.6076
Note: The millilitre equivalences are exact, but cubic-inch and US measures are correct to 5 significant figures.

British apothecaries' volume measures

These measurements were in use from 1824, when the new imperial gallon was defined, but were officially abolished in the United Kingdom on 1 January 1971.[16][17] In the USA, though no longer recommended, the apothecaries' system is still used occasionally in medicine, especially in prescriptions for older medications.[18][19]

Table of British apothecaries' volume units[references 1]
Unit Symbols &
Relative to
metric value*
minim ♏, , m, m., min   59.1938802083 µl
fluid scruple fl ℈, fl s 20 minims 1.18387760416 ml
fluid drachm
(fluid dram, fluidram)
ʒ, fl ʒ, fʒ, ƒ 3, fl dr 3 fluid scruples 3.5516328125 ml
fluid ounce ℥, fl ℥, f℥, ƒ ℥, fl oz 8 fluid drachms 28.4130625 ml
pint O, pt 20 fluid ounces 568.26125 ml
gallon C, gal 8 pints 4.54609 L
* The vinculum over numbers (e.g. 3) represents a repeating decimal.

Mass and weight

In the 19th and 20th centuries, the UK used three different systems for mass and weight:[26]

The troy pound (373.2417216 g) was made the primary unit of mass by the 1824 Act; however, its use was abolished in the UK on 1 January 1879,[27] with only the troy ounce (31.1034768 g) and its decimal subdivisions retained.[28] The Weights and Measures Act 1855 (18 & 19 Victoria C72) made the avoirdupois pound the primary unit of mass[29] 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 (gr) 1/7000 0.06479891
Exactly 64.79891 milligrams.
drachm (dr) 1/256 1.7718451953125
ounce (oz) 1/16 28.349523125
pound (lb) 1 453.59237 0.45359237
Exactly 453.59237 grams by definition.
stone (st) 14 6350.29318 6.35029318
A person's weight is often quoted in stone and pounds in English-speaking countries that use the avoirdupois system, with the exception of the United States and Canada, where it is usually quoted in pounds.
quarter (qtr) 28 12.70058636
One quarter is equal to two stone or a quarter of a hundredweight. The term quarter was also commonly used to refer to a quarter of a pound in a retail context.
hundredweight (cwt) 112 50.80234544
One imperial hundredweight is equal to eight stone. This is the long hundredweight as opposed to the short hundredweight of 100 pounds as used in the United States and Canada.[30]
ton (t) 2240 1016.0469088
As with the US and Canadian[30] systems, twenty hundredweights equal a ton. The imperial hundredweight is 12% greater than the US and Canadian equivalent. The imperial ton (or long ton) is 2,240 pounds, which is much closer to a metric tonne (about 2,204.6 pounds), compared to the short ton of 2,000 pounds (907.185 kg).

Relation to other systems

The imperial system is one of many systems of English units. 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 distinction 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 the Weights and Measures Act 1824 or earlier, or which had fallen out of use by that time, nor to post-imperial inventions such as the slug or poundal.

The US customary system is historically derived from the English units that were in use 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.

Current use of imperial units

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 for most applications; however, use of Imperial units is still widespread amongst the public and all UK roads still primarily use the imperial system except for tonnage on main roads.[31]

The Units of Measurement Regulations 1995 require that all measuring devices used in trade or retail shall display measurements in metric quantities. This has 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 do not currently place 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 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 units of measurement directive (directive 80/181/EEC) had previously permitted the use of supplementary indicators (imperial measurements) until 31 December 2009, but a revision of the directive published on 11 March 2009 permitted their use indefinitely.[32]

The United Kingdom completed its legal partial transition to the metric system (sometimes referred to as "SI" from the French Système International d'Unités) in 1995, with some imperial units still legally mandated for certain applications; draught beer and cider must be sold in pints,[33] road-sign distances must be in yards and miles,[34] length and width (but not weight) restrictions must be in feet and inches on road signs (although an equivalent in metres may be shown as well),[34] and road speed limits must be in miles per hour,[34] therefore instruments in vehicles sold in the UK 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 the UK in the Weights and Measures Act of 1878, the troy ounce still may be used for the weight of precious stones and metals. The original railways (many built in the Victorian era) are 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 Freight Yard, railway speeds are given in both metric and imperial units.

Most British people still use imperial units in everyday life for distance (miles, yards, feet and inches), body weight (stones and pounds for adults, pounds and ounces for babies though use of kilogrammes is increasing) and volume (especially pints and rational fractions thereof).[31][35] Regardless of how people measure their weight or height, these must be recorded in metric officially, for example in medical records. Petrol is occasionally quoted as being so much per gallon (despite having been sold exclusively in litres for nearly three decades). Fuel consumption for vehicles is often discussed in miles per gallon, though official figures always include litres per 100 km equivalents. When sold "draught" in licenced premises, beer and cider is measured out and sold in pints and half-pints. Cow's milk is available in both litre and pint based containers. Non-metric nuts and bolts etc., are available, but usually only from specialist suppliers. Areas of land associated with farming, forestry and real estate are often advertised measured in acres and square feet, but for official government purposes the unit is always hectares and square metres. Office space and industrial units are often advertised in square feet, despite carpet and flooring products being sold in square metres with equivalents in square yards. Steel pipe sizes are sold in increments of inches, while copper pipe is sold in increments of millimetres. Road bicycles are have their frames measured in centimetres, while off-road bicycles have their frames measured in inches.


In 1973, 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 parkade. 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.[36][37][38][39] 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.[40][41] However, there tends to be leniency in regards to fruits and vegetables being priced in imperial units only.[41] Unlike the rest of Canada, metrication in the Francophone province of Quebec has been more fully implemented and metric measures are more consistently used in Quebec than elsewhere in Canada, both officially and among the population.[dubious ]

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. Railways in Canada also continue to use Imperial units.

Imperial units are still used in ordinary conversation. Today, Canadians typically use a mix of metric and imperial measurements in their daily lives. However, the use of the metric and imperial systems varies by age. The older generation mostly uses the imperial system, while the younger generation more often uses the metric system. Newborns are measured in SI at hospitals, but the birth weight and length is also announced to family and friends in imperial units. Drivers' licences use SI 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.[42][43][44][45][46] 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,[47] leading to the erroneous impression that Canadian vehicles are 20% more fuel-efficient than their apparently identical American counterparts for which fuel economy is reported in statute miles per US gallon (neither country specifies which gallon is used). Canadian railways maintain exclusive use of imperial measurements to describe train length (feet), train height (feet), capacity (tons), speed (mph), and trackage (miles).[48]

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 that is already classified in metric is still kept metric (e.g., 9 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.

While units of measure in Canada are legally "Canadian units", they are commonly called imperial units by many Canadians and taught as such in much of Canada.[49]


In Australia, metrication is mostly complete, although imperial units remain current in some areas, generally where international standards remain non-metricated (as in the use of inches for television and computer screens, etc.).

New Zealand

In New Zealand, metrication is mostly complete, although imperial units remain current in some areas, generally where international standards remain non-metricated (as in the use of inches for television and computer screens, etc.).

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 1997 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 and sausages, which are sold in 454-gram (1 lb) packaging. The majority of cars sold pre-2005 feature speedometers with miles per hour.

In many aspects of everyday conversation the imperial system is still used, especially in the terms of height (feet and inches) and body weight (stone and pounds).

Other countries

Some imperial measurements remain in limited use in India, Malaysia, Sri Lanka 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. In India, inches, feet, yards and degrees Fahrenheit are often used in conjunction with their metric counterparts, while area is measured in acres exclusively (hectares are only used in government documents).[50]

Towns and villages in Malaysia with no proper names had adopted the Malay word batu (meaning "rock") to indicate their locations along a main road before the use of metric system (for example, batu enam means "6th mile" or "mile 6"). Many of their names remain unchanged even after the adoption of the metric system for distance in the country.

Petrol is still sold by the imperial gallon in Anguilla, Antigua and Barbuda, Burma, the Cayman Islands, Dominica, Grenada, Montserrat, St Kitts and Nevis and St Vincent and the Grenadines. The United Arab Emirates Cabinet in 2009 issued the Decree No. (270 / 3) specifying that, from 1 January 2010, the new unit sale price for petrol will be the litre and not the gallon. This in line with the UAE Cabinet Decision No. 31 of 2006 on the national system of measurement, which mandates the use of International System of units as a basis for the legal units of measurement in the country.[51][52] Sierra Leone switched to selling fuel by the litre in May 2011.[53]

In October 2011, the Antigua and Barbuda government announced the re-launch of the Metrication Programme in accordance with the Metrology Act 2007, which established the International System of Units as the legal system of units. The Antigua and Barbuda government has committed to a full conversion from the imperial system by the first quarter of 2015.[54]

See also


  • Appendices B and C of NIST Handbook 44
  • 6 George IV chapter 12, 1825 (statute)

External links

  • Fast, simple, easy to use and intuitive unit converter
  • British Weights And Measures Association
  • Canada Weights and Measures Act 1970-71-72
  • General table of units of measure – NIST – pdf
  • How Many? A Dictionary of Units of Measurement
  • Statutory Instrument 1995 No. 1804 Units of Measurement Regulations 1995

es:Sistema anglosajón de unidades fr:Unités de mesure anglo-saxonnes

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