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 (until 1959) 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, although as of 2011 the UK had only partially adopted it.
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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 0.914398416 metres.[2]
| Unit | Relative to previous | Feet | Millimetres | Metres | Notes |
|---|---|---|---|---|---|
| thou (th) | 1⁄12000 | 0.0254 | 0.000 025 4 | 25.4 μm | |
| inch (in) | 1000 thou | 1⁄12 | 25.4 | 0.025 4 | |
| foot (ft) | 12 inches | 1 | 304.8 | 0.3048 | |
| yard (yd) | 3 feet | 3 | 914.4 | 0.9144 | Defined as exactly 0.9144 metres since 1959 |
| chain (ch) | 22 yards | 66 | 20116.8 | 20.1168 | |
| furlong (fur) | 10 chains | 660 | 201.168 | ||
| mile (mi) | 8 furlongs | 5,280 | 1,609.344 | ||
| 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[3] | 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. The commonly accepted definition of a fathom was always 6 feet. The conflict was inconsequential as Admiralty nautical charts designated depths shallower than 5 fathoms in feet 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 | 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. |
| chain | 4 rods | 66 | 20.1168 | 1⁄10 of a 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 | 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 furlong × 1 rod[4] | 10,890 | 40 | 1⁄2560 | 1,011.7141056 | 0.1012 | The rood is also called a rod.[5][6] It is 1,210 square yards. |
| acre | 1 furlong × 1 chain | 43,560 | 160 | 1⁄640 | 4,046.8564224 | 0.4047 | One acre is 4,840 square yards |
| Note: All equivalences are exact except the hectares, which are accurate to four significant figures. | |||||||
In 1824, the United Kingdom 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 0.998859 g/mL weighed in air of density 0.001217 g/mL against weights of density 8.136 g/mL. This 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).[7]
| Unit | Imperial ounce | Imperial pint | 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 whereas the conversions to cubic-inch and US measures are correct to five significant figures. | ||||||
These measurements were in use from 1824, when the new imperial gallon was defined, but fell out of use long before they were officially abolished in 1971.
| Unit | Previous Unit | Metric Value |
|---|---|---|
| minim | ... | 59.1938802 µl |
| fluid scruple | 20 minims | 1.1838776 ml |
| fluid drachm | 3 fluid scruples | 3.5516328 ml |
| fluid ounce | 8 fluid drachms | 28.4130625 ml |
| pint | 20 fluid ounces | 568.26125 ml |
| gallon | 8 pints | 4.54609 l |
In the 19th and 20th centuries the UK used three different systems for mass and weight:[8]
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 6 January 1879, making the Avoirdupois pound the primary unit of mass with only the troy ounce (31.1034768 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 | Pounds | grams | kilograms | Notes |
|---|---|---|---|---|
| grain (gr) | 1⁄7000 | 0.06479891 | Exactly 64.79891 milligrams. | |
| drachm (drc) | 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 | 6,350.29318 | 6.35029318 | A person's weight is often quoted in stone and pounds in English-speaking countries using 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. | |
| ton (t) | 2240 | 1,016.0469088 | As with the US system twenty hundredweights equal a ton. However, since the imperial hundredweight is 12% greater than the US equivalent, the imperial ton (or long ton) is 2,240 pounds, which is very close to a tonne, compared to the short ton of 2,000 pounds (907.185 kg). |
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 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 which 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. Units of length and area are mostly shared between the imperial and US systems, albeit being partially and temporarily 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.
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 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 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.[9]
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 many imperial units still legally mandated for some application; draught beer must be sold in pints,[10] road-sign distances must be in yards and miles,[11] 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),[11] and road speed limits must be in miles per hour,[11] 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 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 Freight Yard, railway speeds are given in both metric and imperial units.
The use of SI units is required by law for the retail sale of some foods and other commodities, whilst imperial units are required for others. Many British people still use imperial units in everyday life for distance (yards, feet and inches), weight (especially stones and pounds) and volume (gallons & pints).[12] Milk is available in both litre and pint based 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 often quoted as being so much per gallon, despite having been sold exclusively in litres for two decades. Likewise, fuel consumption for cars is still almost always in miles per gallon, though official figures always include litres per 100 km equivalents. Fahrenheit equivalents are occasionally given after Celsius in weather forecasts, though this is becoming rare. Threads on non-metric nuts and bolts etc., are sometimes referred to as Imperial, especially in the UK. Areas of land associated with farming or forestry are bought and sold measured in acres, but for official government areas the unit is always hectares.
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 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.[13][14][15][16] 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.[17][18] However, there tends to be leniency in regards to fruits and vegetables being priced in imperial units only.[18] However unlike in the rest of Canada, metrication in the Francophone province of Quebec has been more implemented and metric measures are more consistently used in Quebec than elsewhere in Canada both officially and among the population.
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. Many Canadians use Imperial 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. 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.[19][20][21][22][23] 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,[24] 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).[25]
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). 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.
In Australia, imperial measurements are still encountered peripherally in either spoken or written form.
Rural land areas are sometimes given in acres.[26] Australian beer glass sizes are based on older imperial sizes but rounded to the nearest 5 ml, while some surf reports are given in feet.[27]
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, especially in the terms of height and weight, particularly by the older generation.
Some imperial measurements remain in limited use in India, Malaysia 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).[28]
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, Belize, Burma, the Cayman Islands, Ecuador, Grenada, Guyana, Sierra Leone and the United Arab Emirates. 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 (SI) as a basis for the legal units of measurement in the country.[29][30][31][32][33][34][35][36][37][38] Sierra Leone switched to selling fuel by the litre in May 2011.[39] Antigua and Barbuda will convert to litres before 2015.[40]
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