The system of imperial units or the imperial system (also known as British Imperial or Exchequer Standards of 1825) is the system of units first defined in the British Weights and Measures Act of 1824, which was later refined and reduced. The Imperial Units replaced the Winchester Standards, which were in effect from 1588 to 1825. 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, although some imperial units are still used in the United Kingdom, Canada and other countries formerly part of the British Empire. The imperial system developed from what were first known as English units, as did the related system of United States customary units.
The Weights and Measures Act of 1824 was initially scheduled to go into effect on 1 May 1825. However, the Weights and Measures Act of 1825 pushed back the date to 1 January 1826. The 1824 Act allowed the continued use of pre-imperial units provided that they were customary, widely known, and clearly marked with imperial equivalents.
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.
Imperial apothecaries' measures, based on the imperial pint of 20 fluid ounces, were introduced by the publication of the London Pharmacopoeia of 1836, the Edinburgh Pharmacopoeia of 1839, and the Dublin Pharmacopoeia of 1850. The Medical Act of 1858 transferred to The Crown the right to publish the official pharmacopoeia and to regulate apothecaries' weights and measures.
|Unit||Relative to previous||Feet||Millimetres||Metres||Notes|
|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||
|chain (ch)||22 yards||66||116.820||20.1168|
|furlong (fur)||10 chains||660||201.168||
|mile (ml)||8 furlongs||5280||609.3441||
|league (lea)||3 miles||84015||828.0324||
|fathom (ftm)||2.02667 yards||6.08||853.1841||1.853184||
|nautical mile||10 cables||6080||853.1841|
|Gunter's survey units (17th century onwards)|
|Unit|| Relation to
units of length
|Square feet||Square rods||Square miles||Square metres||Hectares||Notes|
|perch||1 rod × 1 rod||272.25||1||1⁄400102||8526425.292||5290.002||
|rood||1 furlong × 1 rod||89010||40||1⁄2560||011.71410561||0.1012||
|acre||1 furlong × 1 chain||56043||160||1⁄640||046.85642244||0.4047||
|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 859 g/ml weighed in air of density 0.998217 g/ml against weights of density 0.001 g/ml, which works out to 8.136096 l or 4.546 cu in. The Weights and Measures Act of 1985 switched to a gallon of exactly 277.419809 l (approximately 4.546 cu in). 277.4194
|Millilitres||Cubic inches||US ounces||US pints|
|fluid ounce (fl oz)||1||1⁄20||28.4130625||1.7339||0.96076||0.060047|
|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 1826, when the new imperial gallon was defined, but were officially abolished in the United Kingdom on 1 January 1971. In the USA, though no longer recommended, the apothecaries' system is still used occasionally in medicine, especially in prescriptions for older medications.
|Unit|| Symbols &
| Relative to
|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||6328125 ml3.551|
|fluid ounce||℥, fl ℥, f℥, ƒ ℥, fl oz||8 fluid drachms||0625 ml28.413|
|pint||O, pt||20 fluid ounces||25 ml568.261|
|gallon||C, gal||8 pints||4.54609 l|
Mass and weight
- 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 troy pound (7216 g) was made the primary unit of mass by the 1824 Act; however, its use was abolished in the UK on 1 January 1879, 373.241 with only the troy ounce (4768 g) and its 31.103decimal subdivisions retained. The Weights and Measures Act 1855 (18 & 19 Victoria C72) made the avoirdupois pound the primary unit of mass. In all the systems, the fundamental unit is the pound, and all other units are defined as fractions or multiples of it.
|grain (gr)||1⁄7000||798910.064||Exactly 91 milligrams. 64.798|
|pound (lb)||1||37453.592||592370.453||Exactly 37 grams by definition. 453.592|
|stone (st)||14||350.293186||293186.350||The plural stone is often used when providing a weight (e.g. "this sack weighs 8 stone"). A person's weight is often quoted in stones 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 (qr or qtr)||28||5863612.700||One quarter is equal to two stones 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||3454450.802||One imperial hundredweight is equal to eight stones. This is the long hundredweight as opposed to the short hundredweight of 100 pounds as used in the United States and Canada.|
|ton (t)||2240||016.04690881||As with the US and Canadian 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 pounds, which is much closer to a metric 2240tonne (about 204.6 pounds), compared to the short ton of 22000 pounds (907.185 kg).|
|slug (slug)||0485632.174||593.9029414||9029414.593||The slug, a unit associated with imperial and US customary systems, is a mass that accelerates by 1 ft/s2 when a force of one pound (lbF) is exerted on it.|
Although the 1824 act defined the yard and pound by reference to the prototype standards, it also defined the values of certain physical constants, to make provision for re-creation of the standards if they were to be damaged. For the yard, the length of a pendulum beating seconds at the latitude of Greenwich at Mean Sea Level in vacuo was defined as 93 inches, and, for the pound, the mass of a cubic inch of distilled water at an 39.013atmospheric pressure of 30 inches of mercury and a temperature of 62° Fahrenheit was defined as 252.458 grains. However, following the destruction of the original prototypes in the 1834 Houses of Parliament fire, it proved impossible to recreate the standards from these definitions, and a new Weights and Measures Act (18 & 19 Victoria. Cap. 72) was passed in 1855 which permitted the recreation of the prototypes from recognized secondary standards.
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 some imperial units
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 some applications; however, use of imperial units is widespread amongst the public, and all UK roads use the imperial system except for tonnage on main roads.
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 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.
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, road-sign distances must be in yards and miles, 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), and road speed limits must be in miles per hour, 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 metric. 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 in some cases (especially pints of milk, beer, and rational fractions thereof but rarely for canned or bottled soft drinks or petrol). The height of horses in some English-speaking countries, including Australia, Canada, the Republic of Ireland, the United Kingdom and the United States is usually measured in hands, standardized to 4 inches (101.6 mm). Regardless of how people measure their weight or height, these must be recorded in metric officially, for example in medical records. 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 licensed premises, beer and cider must be measured out and sold in pints and half-pints. Cow's milk is available in both litre- and pint-based containers in supermarkets and shops. Non-metric nuts and bolts etc., are available, but usually only from specialist suppliers and mostly for repair of legacy equipment. 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 usually 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 have their frames measured in centimetres, while off-road bicycles have their frames measured in inches due to US influence in that market. The size (diagonal) of television and computer monitor screens is always denominated in inches. Food sold by length or width e.g. pizzas or sandwiches, are generally sold in inches. Clothing is always sized in inches, with the metric equivalent often shown as a small supplementary indicator. Gas is usually billed by the cubic foot, whereas electricity is billed by the kilowatt-hour.
Many pre-packaged foods show both metric and imperial measures e.g. Thorntons chocolates and Typhoo tea, however it is also common to see imperial pack sizes with metric only labels e.g. a 1 lb tin of Lyle's Golden Syrup is always labelled 454 g with no imperial indicator. Similarly most jars of jam and packs of sausages are labelled 454 g with no imperial indicator.
India's conversion to the metric system from the imperial system occurred in stages between 1955 and 1962. The metric system in weights and measures was adopted by the Indian Parliament in December 1956 with the Standards of Weights and Measures Act, which took effect beginning 1 October 1958. The Indian Coinage Act was passed in 1955 by the Government of India to introduce decimal coinage in the country. The new system of coins became legal tender on April 1957, where the rupee consists of 100 paise. For the next five years, both the old and new systems were legal. In April 1962, all other systems were banned. This process of metrication is called "big-bang" route, which is to simultaneously outlaw the use of pre-metric measurement, metricise, reissue all government publications and laws, and change education systems to metric
Today all official measurements are made in the metric system. However, in common usage some older Indians may still refer to imperial units. Some measurements, such as the heights of mountains, are still recorded in feet. Additionally, the Indian numbering system of crores and lacs is used alongside otherwise metricated currency units, while tyre rim diameters are still measured in inches, as used worldwide. Road widths are popularly measured in feet but official documents use metres. Body temperature is still sometimes measured in degrees Fahrenheit. Industries like the construction and the real estate industry still use both the metric and the imperial system though it is more common for sizes of homes to be given in square feet and land in acres. Bulk cotton is sold by the candy (0.35 imperial tons, or 355.62 kg) or the bale (170 kg) .
In Standard Indian English, as in Australian, Singaporean, and British English, metric units such as the litre (liter), metre (meter), and metric tonne (ton) utilise the traditional spellings brought over from French, which differ from those used in the United States and the Philippines. The imperial long ton is invariably spelt with one 'n'. (See English in the Commonwealth of Nations for more information)
- The Chinese units of measurement of the Qing Empire (no longer in widespread use in mainland China);
- British imperial units; and
- The metric System.
In 1976 the Hong Kong Government started the conversion to the metric system, and as of 2012 measurements for government purposes, such as road signs, are almost always in metric units. However, all three systems are officially permitted for trade, and in the wider society a mixture of all three systems prevails.
The Chinese system's most commonly used units for length are 里 (li), 丈 (tseung/cheung), 尺 (tsek/chek), 寸 (tsun/chun), 分 (fen/fan) in descending scale order. These units are now rarely used in daily life, the imperial and metric systems being preferred. The imperial equivalents are written with the same basic Chinese characters as the Chinese system. In order to distinguish between the units of the two systems, the units can be prefixed with "Ying" (Chinese: 英) for the Imperial system and "Wa" (Chinese: 華) for the Chinese system. In writing, derived characters are often used, with an additional 口 (mouth) radical to the left of the original Chinese character, for writing imperial units. The most commonly used units are the mile or "li" (Chinese: 哩), the yard or "ma" (Chinese: 碼), the foot or "chek" (Chinese: 呎), and the inch or "tsun" (Chinese: 吋).
The traditional measure of flat area is the square foot (Chinese: 方呎, 平方呎) of the imperial system, which is still in common use for real estate purposes. The measurement of agricultural plots and fields, however, is traditionally conducted in 畝 (mau) of the Chinese system.
For the measurement of volume, Hong Kong officially uses the metric system, though the gallon (加侖, ka-lun) is also occasionally used.
During 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 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. 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. 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. 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 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).
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., .204 Ruger, .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., 9×19mm). 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, though altitude is still measured in imperial feet in keeping with the international standard.
Metrication in Australia has largely ended the use of imperial units, though for particular measurements (such as flight altitudes and nominal sizes of computer and television screens) international usage of imperial units is still followed. In licensed venues, draught beer and cider is sold in glasses and jugs with sizes based on the imperial fluid ounce though rounded to the nearest 5 ml.
Although New Zealand completed metrication in the 1970s, a study of university students undertaken in 1992 found a continued use of imperial units for birth weight and human height alongside metric units.
The aviation industry is one of the last major users of the old imperial system: altitude and airport elevation is measured in feet. All other aspects (fuel quantity, aircraft weight, runway length, etc.) use metric.
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 as the primary unit, but with a kilometres per hour display as well.
Some imperial measurements remain in limited use in Malaysia, Philippines, Sri Lanka and South Africa. 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 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, Myanmar, 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. Sierra Leone switched to selling fuel by the litre in May 2011.
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.
|Wikimedia Commons has media related to British Imperial units.|
- References for the Table of British apothecaries' volume units: Unit column;:C-7 Symbols & abbreviations column;:C-5,C-17–C-18 Relative to previous column;:C-7 Exact metric value column — fluid ounce, pint and gallon, all other values calculated using value for fluid ounce and the Relative to previous column's values.
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#Kerosene per US Gallon (per Imperial gallon)#Gasoline (Regular)(per imperial Gallon)# Gasoline (Premium) (per Imperial Gallon)#Diesel (per Imperial Gallon)
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