Imperial units

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Apothecaries' units[edit]

Imperial standards of length 1876 in Trafalgar Square, London.






Table of length equivalent units
Unit Relative to previous Feet Millimetres Metres Notes
thou (th) 112000 0.0254 0.0000254
Also 25.4 μm
inch (in) 1000 thou 112 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 metres by the International yard and pound agreement of 1959
chain (ch) 22 yards 66 20116.8 20.1168
100 links, 4 rods or 110 of a furlong. The distance between the two wickets on a cricket pitch
furlong (fur) 10 chains 660 201.168
220 yards
mile (ml) 8 furlongs 5280 1609.344
1760 yards or 80 chains
league (lea) 3 miles 15840 4828.032
No longer an official unit in any nation.
Maritime units
fathom (ftm) 2.0266 yards 6.08 1828.8 1.8288
The British Admiralty in practice used a fathom as 6 feet. This was despite its being 11000 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 informally as 100 fathoms.
nautical mile 10 cables 6080 1853.184
Used for measuring distances at sea. Until the adoption of the international definition of 1852 metres in 1970, the British nautical (Admiralty) mile was defined as 6080 feet.[13]
Gunter's survey units (17th century onwards)
link 7.92 inches 66100 201.168 0.201168
1100 of a chain and 11000 of a furlong
rod 25 links 664 5029.2 5.0292
The rod is also called pole or perch and equal to 5 12 yards


Unit Relation to
units of length
Square feet Square rods Square miles Square metres Hectares Notes
perch 1 rod × 1 rod 272.25 1 1102400 25.29285264 0.002529
rood 1 furlong × 1 rod[14] 10890 40 12560 1011.7141056 0.1012
The rood is 1210 square yards.
acre 1 furlong × 1 chain 43560 160 1640 4046.8564224 0.4047
One acre is 4840 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     120     28.4130625 1.7339 0.96076 0.060047
gill (gi) 5     14     142.0653125 8.6694 4.8038 0.30024
pint (pt) 20     1     568.26125 34.677 19.215 1.2009
quart (qt) 40     2     1136.5225 69.355 38.430 2.4019
gallon (gal) 160     8     4546.09 277.42 153.72 9.6076

British apothecaries' volume measures[edit]


Table of British apothecaries' volume units[nb 1]
Unit Symbols &
Relative to
metric value[note 1]
fluid scruple fl ℈, fl s 20 minims 1.18387760416 ml
fluid drachm
(fluid dram, fluidram)
pint O, pt 20 fluid ounces 568.26125 ml
gallon C, gal 8 pints 4.54609 l
  1. ^ The vinculum over numbers (e.g. 3) represents a repeating decimal.

Mass and weight[edit]



Table of mass units
Unit Pounds Grams Kilograms Notes
grain (gr) 17000 0.06479891
Exactly 64.79891 milligrams.
drachm (dr) 1256 1.7718451953125
ounce (oz) 116 28.349523125
pound (lb) 1 453.59237 0.45359237
Exactly 453.59237 grams by definition.
stone (st) 14 6350.29318 6.35029318
The plural stone is often used when providing a weight (e.g. "this sack weighs 8 stone").[30] 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 12.70058636
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 50.80234544
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.[31]
ton (t) 2240 1016.0469088
As with the US and Canadian[31] 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 2240 pounds, which is much closer to a metric tonne (about 2204.6 pounds), compared to the short ton of 2000 pounds (907.185 kg).
Gravitational units
slug (slug) 32.17404856 14593.90294 14.59390294
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.[32]
F  = ma (Newton's second law)
1 lbF  = 1 slug × 1 ft/s2 (as defined above)
1 lbF  = 1 lbm × g (by definition of the pound force)
g  32.17404856 ft/s2
1 slug  32.17404856 pounds

Natural equivalents[edit]


Relation to other systems[edit]

English units of Length




Current use of some 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.[33]


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,[35] road-sign distances must be in yards and miles,[36] 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),[36] and road speed limits must be in miles per hour,[36] 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.[37] 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,[38] 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).[33][39] The height of horses in some English-speaking countries, including Australia,[1] 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. 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. 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.

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.





Hong Kong[edit]


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,[42] and in the wider society a mixture of all three systems prevails.

The Chinese system's most commonly used units for length are (li), (tseung/cheung), (tsek/chek), (tsun/chun), (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: ) for the Imperial system and "Wa" (Chinese: ) for the Chinese system. In writing, derived characters are often used, with an additional (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: ), the yard or "ma" (Chinese: ), the foot or "chek" (Chinese: ), and the inch or "tsun" (Chinese: ).







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[56] 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[citation needed] 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.

New Zealand[edit]





Other countries[edit]

Some imperial measurements remain in limited use in Malaysia, 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.


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.[58][59] Sierra Leone switched to selling fuel by the litre in May 2011.[60]


See also[edit]




  7. ^ "A Translation of the Pharmacopoeia of the Royal College of Physicians of London, 1836.: With ...". 
  14. ^ "Appendix C: General Tables of Units of Measurements" (PDF). NIST. Archived from the original (PDF) on 26 November 2006. Retrieved 4 January 2007. 
  15. ^ "imperial gallon". 
  16. ^ "The Weights and Measures (Equivalents for dealings with drugs) Regulations 1970". 
  17. ^ "Museum of the Royal Pharmaceutical Society, London, Information Sheet: 11" (PDF). 
  30. ^ "Definition of stone in English from the Oxford dictionary". Oxford University Press. Retrieved 2015-11-25. 
  31. ^ a b Weights and Measures Act
  32. ^ "Wolfram-Alpha: Computational Knowledge Engine". 
  35. ^ "BusinessLink: Weights and measures: Rules for pubs, restaurants and cafes". Department for Business, Innovation & Skills. Archived from the original (online) on 2011-07-20. Retrieved 24 August 2009. 
  37. ^ "Facts & Figures". Transport for London. Transport for London. Retrieved 31 January 2016. 
  38. ^ "BMI healthy weight calculator". National Health Service. Retrieved 25 November 2015. 
  39. ^ "In praise of ... metric measurements". The Guardian. London. 1 December 2006. Retrieved 5 May 2010. 
  40. ^ a b "".  External link in |title= (help)
  43. ^ "Weights and Measures Act: Canadian units of measure". Justice Canada. Archived from the original on 5 June 2011. Retrieved 14 November 2007. 
  46. ^ "A Canadian compromise". CBC. Retrieved 12 March 2008. 
  47. ^ "A Canadian compromise". CBC. Retrieved 11 March 2008. 
  50. ^ [1][dead link]
  51. ^ [2][dead link]
  52. ^ [3][dead link]
  53. ^ "Home Hardware - Building Supplies - Building Materials - Fence Products". 
  54. ^ Canada, Government of Canada, Natural Resources. "Fuel Consumption Ratings Search Tool - Conventional Vehicles". 
  55. ^ Canada, Government of Canada, Transportation Safety Board of. "Railway Investigation Report R96W0171". 
  56. ^ "Canadian Aviation Regulations, Langley Flying School. Canadian Airport Organization, Runways and Taxiways, Mandatory Instruction Signs, Manoeuvring Area, Windsocks, Airport Circuit Organization in Canada, Uncontrolled Airports, Rules for Joining the Circuit in Canada, Airports with Mandatory Radio Frequency, Controlled Airports in Canada, ATC clearances and instructions, Canadian Airspace Organization, Canadian Class C Airspace, Canadian Class D Airspace, Restricted Airspace in Canada, Airway organization in Canada, VHF Airways, Control Zones in Canada, VFR Weather Minimum in Canada, Special VFR Clearances in Canada, VFR Over-the-top, Aircraft cruising altitude rules in Canada, Altimeter Rules in Canada, VFR Flight Plans in Canada, NOTAMs in Canada, National Security rules for aircraft in Canada, Emergency Locator Transmitters, Priority Aircraft Radio Communications in Canada, Transponder Requirements in Canada, Aircraft Fuel Requirements in Canada, Aircraft Passenger Briefings in Canada, Pilot Privileges in Canada, Aircraft Wake Turbulence, Aircraft Jet Blast Hazard, Aircraft passenger seat requirements in Canada, Aircraft Oxygen requirements in Canada, Aircraft Documents required in Canada, Aircraft Equipment requirements in Canada, Aircraft Icing, Minimum flight altitude in Canada, Rules of the Air in Canada,". 

External links[edit]