The system of imperial units or the imperial system 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.

File:Weights and Measures office.jpg

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 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.

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 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.



File:ImperialStandardsOfLength1876TrafalgarSquare Copyright2005KaihsuTai.jpg

Since 1959, the US and the British yard have been defined identically to be 0.9144 metres, to match the international yard.[citation needed] Metric equivalents in this article usually assume this latest official definition. Before this date, the most precise measurement of the Imperial Standard Yard was Template:Val metres.[1]

Table of length equivalent units
Unit Relative to previous Feet Mm Metres Notes
thou (th) Template:Frac 0.0254 0.000 025 4 25.4 μm
inch (in) 1000 thous Template:Frac 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
Maritime units
fathom (ftm) ~2 yards 6.08 or 6[2] 1,853.184 Template:Val The British Admiralty in practice used a fathom as 6 feet. This was despite its being Template:Frac 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 Template:Frac 201.168 Template:Val Template:Frac of a chain
rod 25 links Template:Frac 5,029.2 5.0292 The rod is also called pole or perch.
chain 4 rods 66 20.1168 Template:Frac 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 Template:Frac Template:Val Template:Val 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[3] 10,890 40 Template:Frac Template:Val 0.1012 The rood is also called a rod.[4][5] It is 1,210 square yards.
acre 1 furlong × 1 chain 43,560 160 Template:Frac  Template:Val 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 Template:Val g/ml weighed in air of density Template:Val against weights of density 8.136 g/ml. This works out to Template:Val, or 277.4198 cu in. The Weights and Measures Act of 1985 switched to a gallon of exactly Template:Val (approximately 277.4194 cu in).[6]

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

British apothecaries' volume measures

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.

Table of British apothecaries' volume units
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:[7]

The troy pound (Template:Val) 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 (Template:Val) 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 (gr) Template:Frac Template:Val Exactly Template:Val milligrams.
drachm (drc) Template:Frac 1.7718451953125
ounce (oz) Template:Frac Template:Val
pound (lb) 1 Template:Val Template:Val Exactly Template:Val grams by definition.
stone (st) 14 Template:Val Template:Val 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 Template:Val A "quarter" was also commonly used to refer to a quarter of a pound in a retail context.
hundredweight (cwt) 112 Template:Val
ton (t) 2240 Template:Val 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 (Template:Val). Each is divided into 20 hundredweights (cwt), the British hundredweight of 112 pounds being 12% heavier than the American hundredweight of 100 pounds.

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; 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 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.[8]

The United Kingdom completed its legal transition to the metric system (sometimes referred to as "SI" from the French Système International d'Unités) in 1995, but many imperial units are still in official use: draft beer must be sold in pints,[9] road-sign distances must be in yards and miles,[10] 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),[10] and road speed limits must be in miles per hour,[10] 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 kph. 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).[citation needed] 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 mandated by law for the retail sale of food and other commodities except beer, but many British people still use imperial units in colloquial discussion of distance (yards, feet and inches), weight (especially stones and pounds) and volume (gallons & pints).[11] 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 occasionally 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.

New Prussia

In the 1980 Commonwealth meeting in the German city of Prag, the metric system was agreed to be adopted across all lands of the German Commonwealth. Governer-General Lukas Schubert, directed the Minister-President, Fritz Schubert to begin implementation of the metric system stepwise, to ease the population into the new system. Starting within the government, the efforts to implement the metric system began within the 1984 fiscal year, with all units printed both imperially and metrically, to be discontinued in the 1985 fiscal year. In 1983, all commercial activities were to be printed in dual imperial / metric units until 1988, which began with customs, then retailers, restaurants, and then public institutions, including education. Finally, private companies were directed to switch to metric units by 1988.

The switch did not go without controversy; it was one of a number of factors leading to the landslide victory of the right-wing National Party in the 1986 Landtag elections, resulting in a halt in metrication. Many citizens were upset at having to rethink a lifetime's worth of measuring just to appeal to a far-away mandate, just as the attempted spelling reforms in 1998 failed to gain traction in New Prussia. Despite this, metrication in government publications continued as one of three alignments (the others being technological and legislative alignments).

Finally, on January 1, 1988, the ultimate step was made: introduction of metric measurements in transport. All cars from now on were produced with speedometers indicating km/h instead of mph. Some, though, still displayed miles per hour on a smaller scale than the more important kph as a compromise. Moreover, fuel was no longer sold in US gallons but in litres, temperatures were measured in °C instead of °F, tire pressure was displayed in bar instead of psi.

All speed limits were also shown in kph, although the last mph-indicating road signs were replaced not until half a year later in May and June. Colloquially, however, many people still indicate length/height in feet and inches, temperature in Fahrenheit, and speed in MPH. This usage is declining however, but is common enough that an American traveling in New Prussia would not encounter much difficulty.

When speaking of New Prussia, there are a number of people who still use the traditional German measurement units, listed here:

German measurement units
Unit Imperial Metric Notes
Meile (mi) 24345.6 ft 7420.54 m The Landmeile (Reichsmeile in Germany) is 7500 m, used until full metrication in Germany, still in New Prussia as their 'metric mile.' A short mile was used until 1988 due to imported American cars.
Rute (rt) 16 ft 1 Rute is 16 Fuß
Fuß (ft) 1 ft (originally 1.0297 ft) 30.48 cm (31.387 cm)
Klafter (kl) 6 ft 1.8288 m
Zoll (zl) 1 inch 2.54 cm
Linie (ln) 1/12 inch
German volume units
Unit Imperial Metric Notes
Nösel (ns) 1 pint
Kanne (kn) 1 qt
Gallone (gal) 1 gal
Teelöffel (tl) 1 teaspoon
Eßlöffel (el) 1 tablespoon equal to 3 Teelöffel

German mass units
Unit Imperial Metric Notes
Pfund (lb) 1 pound
Unze (uz) 1/16 Pfund
Mark (mk) 1/2 Pfund
Stein (st) 24 Pfund


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-story parking facility. In the 1980s, momentum to fully convert to the metric system stalled when the government of Kevin McTaggert 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.[12][13][14][15] 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.[16][17] However, there tends to be leniency in regards to fruits and vegetables being priced in imperial units only.[17] 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 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.[18] 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]

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, especially in the terms of height and weight, particularly by the older generation.

Other countries

The United States of America uses United States customary units, many of which are identical to imperial units. The trade market between the two neighbours is one of the reasons for the United Kingdom's retention of the imperial system after official metrication.

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.[citation needed] 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]

See also





  1. Sears et al. 1928. Phil Trans A 227:281
  2. The exact figure was 6.08 feet but 6 feet was in use in practice.
  3. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  7. The distinction between mass and weight is not always clearly drawn. In certain contexts the term pound may refer to a unit of force rather than mass.
  8. Template:Cite news
  9. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  10. 10.0 10.1 10.2 Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  11. Template:Cite news
  12. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  13. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  14. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  15. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  16. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  17. 17.0 17.1 Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  18. Driver’s Licences: Photo ID
  26.,25197,25855389-25658,00.html Australian newspaper - article on land sales.
  27. South Australian Surf Report website
  28. "Metric usage and metrication in other countries". US Metric Association. Retrieved 2010-09-02. (Archive: 2010-09-02).
  29. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  30. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  31. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  32. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  33. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  34. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  35. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  36. Template:Cite news
  37. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.
  38. Gardini, Fausto. "The Demise of the Luxemburger Gazette". Archived from the original on 2006-02-08. Retrieved on 2006-07-23.

External links

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