Imperial units

From Wikipedia, the free encyclopedia
Jump to: navigation, search
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 former Weights and Measures office in Seven Sisters, London.

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; however some Imperial units are still used in the United Kingdom and Canada. This system developed from what was first known as English units.


The Weights and Measures 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[edit]

Imperial standards of length 1876 in Trafalgar Square, London.


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 The Crown the right to publish the official pharmacopoeia and to regulate apothecaries' weights and measures.[10]




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 20,116.8 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
1,760 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
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 × 1 rod 272.25 1 1/102400 25.29285264 0.002529
rood 1 furlong × 1 rod[14] 10,890 40 1/2560 1,011.7141056 0.1012
The rood 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


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

British apothecaries' volume measures[edit]

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 ♏, Mx, a symbol for minim in the apothecaries' system.svg, 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
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[edit]

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 6,350.29318 6.35029318
quarter (qr) 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 1,016.0469088
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).

Natural equivalents[edit]

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 39.013 93 inches, and, for the pound, the mass of a cubic inch of distilled water at an atmospheric pressure of 30 inches of mercury and a temperature of 62° Fahrenheit was defined as 252.458 grains.[2]

Relation to other systems[edit]

English units of Length




Current use of imperial units[edit]


United Kingdom[edit]

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).[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.

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 have their frames measured in centimetres, while off-road bicycles have their frames measured in inches. The size (diagonal) of television and computer monitor screens is denominated in inches.


A one U.S. gallon gas can purchased near the U.S.-Canada border. It shows equivalences in imperial gallons and litres.

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 ]




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[49] in keeping with the international standard.

Australia and New Zealand[edit]

In Australia and 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, feet for aviation altitudes,[citation needed] etc.).



Other countries[edit]

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, rarely 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 often still measured in acres though hectares are used in government documents; the Celsius scale is used for weather readings and forecasts, but the Fahrenheit scale is often used for body temperatures.[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]


See also[edit]


  7. ^ A Translation of the Pharmacopoeia of the Royal College of Physicians of London, 1836
  10. ^ Great Britain (1858). A collection of the public general statutes passed in the ... year of the reign of .... Printed by G. W. Eyre and W. Spottiswoode, Printers to the Queen. p. 306. Retrieved 29 July 2012. 
  11. ^ Sears et al. 1928. Phil Trans A, 227:281.
  12. ^ a b The exact figure was 6.08 feet, but 6 feet was in use in practice. 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.
  14. ^ "Appendix C: General Tables of Units of Measurements" (PDF). NIST. Retrieved 4 January 2007. [dead link]
  15. ^
  16. ^ The Weights and Measures (Equivalents for dealings with drugs) Regulations 1970
  17. ^ Museum of the Royal Pharmaceutical Society, London, Information Sheet: 11
  26. ^ 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.
  27. ^ Great Britain (1878). Statutes at large .... p. 308. Retrieved 12 September 2012. 
  30. ^ a b Weights and Measures Act
  31. ^ a b Jon Kelly (21 December 2011). "Will British people ever think in metric?". BBC. Retrieved 13 March 2012. "...but today the British remain unique in Europe by holding onto imperial weights and measures. ...the persistent British preference for imperial over metric is particularly noteworthy..." 
  33. ^ "BusinessLink: Weights and measures: Rules for pubs, restaurants and cafes" (online). Department for Business, Innovation & Skills. Archived from the original on 2011-07-20. Retrieved 24 August 2009. 
  35. ^ "In praise of ... metric measurements". The Guardian (London). 1 December 2006. Retrieved 5 May 2010. 
  36. ^ "Weights and Measures Act: Canadian units of measure". Justice Canada. Retrieved 14 November 2007. [dead link]
  37. ^ "11". Guide to Food Labelling and Advertising. Canadian Food Inspection Agency. Retrieved 1 December 2007. 
  39. ^ "A Canadian compromise". CBC. Retrieved 12 March 2008. 
  40. ^ "A Canadian compromise". CBC. Retrieved 11 March 2008. 
  42. ^ Britishweights And Measures Association
  43. ^ [1][dead link]
  44. ^ [2][dead link]
  45. ^ [3][dead link]
  46. ^ Home Hardware - Building Supplies - Building Materials - Fence Products
  47. ^ Fuel Consumption Ratings | Office of Energy Efficiency
  48. ^ Transportation Safety Board | Home
  49. ^
  50. ^ "Metric usage and metrication in other countries". US Metric Association. Retrieved 2010-09-02. (Archive: 2 September 2010).
  51. ^ "The Government of Grenada – The Ministry of Agriculture". Archived from the original on 24 March 2008. Retrieved 15 January 2008. "he price of gasoline at the pumps was fixed at EC$7.50 per imperial gallon..." , "Belize Ministry of Finance::FAQ". Belize Ministry of Finance. Retrieved 15 January 2008. "#Kerosene per US Gallon (per Imperial gallon)#Gasoline (Regular)(per Imperial Gallon)# Gasoline (Premium) (per Imperial Gallon)#Diesel (per Imperial Gallon)" 
  52. ^ "The High Commission Antigua and Barbuda". Retrieved 15 January 2008. , "FuelPrices2005" (PDF). German Technical Cooperation. p. 96. Retrieved 15 January 2008. 
  53. ^ Sierra Leone Embassy to the United States INTRODUCTION OF THE METRIC SYSTEM AND THE PRICE OF PETROLEUM PRODUCTS Retrieved 23 October 2011.
  54. ^ "Minister Lovell Addresses Metric Conversions". CARIBARENA Antigua. 18 October 2011. Retrieved 23 October 2011. 
  1. ^ References for the Table of British apothecaries' volume units: Unit column;[20][21]:C-7[22] Symbols & abbreviations column;[18][19][20][21]:C-5,C-17–C-18[22][23][24] Relative to previous column;[20][21]:C-7 Exact metric value column — fluid ounce, pint and gallon,[25] all other values calculated using value for fluid ounce and the Relative to previous column's values.

External links[edit]