The system of imperial units or the imperial system (also known as British Imperial) 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. The imperial system developed from what were first known as English units, as did the separate system of United States customary units.
- 1 Implementation
- 2 Units
- 3 Natural equivalents
- 4 Relation to other systems
- 5 Current use of imperial units
- 6 See also
- 7 References
- 8 External links
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 The Crown the right to publish the official pharmacopoeia and to regulate apothecaries' weights and measures.
Metric equivalents in this article usually assume the latest official definition. Before this date, the most precise measurement of the Imperial Standard Yard was 0.914398416 metres.
|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||20,116.8||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||
|fathom (ftm)||~2 yards||6||1,828.8||1.8288|
|nautical mile||10 cables||6,080||1,853.184||
|Gunter's survey units (17th century onwards)|
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||10,890||40||1/2560||1,011.7141056||0.1012||
|acre||1 furlong × 1 chain||43,560||160||1/640||4,046.8564224||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 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).
|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 1824, 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.
|minim||♏, , m, m., min||59.1938802083 µl|
|fluid scruple||fl ℈, fl s||20 minims||1.18387760416 ml|
(fluid dram, fluidram)
|ʒ, fl ʒ, fʒ, ƒ 3, fl dr||3 fluid scruples||3.5516328125 ml|
|fluid ounce||℥, fl ℥, f℥, ƒ ℥, fl oz||8 fluid drachms||28.4130625 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
In the 19th and 20th centuries, the UK used three different systems for 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 (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, with only the troy ounce (31.1034768 g) and its decimal 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.
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. 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 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 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.
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.
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 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.
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). 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.
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. 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 ]
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., 0.204 Ruger, 0.17 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 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, though altitude is still measured in imperial feet in keeping with the international standard.
Australia and New Zealand
|This section requires expansion with: About as much info as the Canadian section. (October 2013)|
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, etc.).
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 Canada, 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. 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.
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. 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.|
- Britannica Educational Publishing (1 August 2010). The Britannica Guide to Numbers and Measurement. The Rosen Publishing Group. p. 241. ISBN 978-1-61530-218-5. Retrieved 10 December 2011.
- Great Britain (1824). The statutes of the United Kingdom of Great Britain and Ireland (1807-1865). His Majesty's statute and law printers. pp. 339–354. Retrieved 31 December 2011.
- Great Britain; William David Evans; Anthony Hammond; Thomas Colpitts Granger (1836). A collection of statutes connected with the general administration of the law: arranged according to the order of subjects. W. H. Bond. pp. 306–27. Retrieved 31 December 2011.
- Edinburgh medical and surgical journal. A. and C. Black. 1824. p. 398. Retrieved 29 July 2012.
- Ireland; Butler, James Goddard; Ball, William (barrister.) (1765). The Statutes at Large, Passed in the Parliaments Held in Ireland: From the twenty-third year of George the Second, A.D. 1749, to the first year of George the Third, A.D. 1761 inclusive. Boulter Grierson. p. 852. Retrieved 29 July 2012.
- Gray, Samuel Frederick (1836). A supplement to the Pharmacopœia and treatise on pharmacology in general: including not only the drugs and preparations used by practitioners of medicine, but also most of those employed in the chemical arts : together with a collection of the most useful medical formulæ .... Longman, Rees, Orme, Brown, Green, and Longman. p. 516. Retrieved 29 July 2012.
- A Translation of the Pharmacopoeia of the Royal College of Physicians of London, 1836
- The Pharmacopoeia of the Royal College of Physicians of Edinburgh. Adam and Charles Black and Bell and Bradfute. 1839. pp. xiii–xiv. Retrieved 29 July 2012.
- Royal College of Physicians of Dublin; Royal College of Physicians of Ireland (1850). The pharmacopœia of the King and queen's college of physicians in Ireland. Hodges and Smith. p. xxii. Retrieved 29 July 2012.
- 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.
- Sears et al. 1928. Phil Trans A, 227:281.
- 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.
- The nautical mile 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).
- "Appendix C: General Tables of Units of Measurements" (PDF). NIST. Retrieved 4 January 2007.[dead link]
- The Weights and Measures (Equivalents for dealings with drugs) Regulations 1970
- Museum of the Royal Pharmaceutical Society, London, Information Sheet: 11
- Zentz, Lorraine C. (2010). "Chapter 1: Fundamentals of Math — Apothecary System". Math for Pharmacy Technicians. Sudbury, MA: Jones & Bartlett Learning. pp. 7–8. ISBN 978-0-7637-5961-2. OCLC 421360709. Retrieved 6 July 2012.
- Boyer, Mary Jo (2009). "UNIT 2 Measurement Systems: The Apothecary System". Math for Nurses: A Pocket Guide to Dosage Calculation and Drug Preparation (7th ed.). Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams & Wilkins. pp. 108–9. ISBN 978-0-7817-6335-6. OCLC 181600928. Retrieved 6 July 2012.
- Royal College of Physicians of Dublin (1850). "Weights and Measures". The Pharmacopœia of the King and Queen's College of Physicians in Ireland. Dublin: Hodges and Smith. p. xlvi. OCLC 599509441. Retrieved 6 July 2012.
- National Institute of Standards and Technology (October 2011). Butcher, Tina; Cook, Steve; Crown, Linda et al. eds. "Appendix C – General Tables of Units of Measurement" (PDF). Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices. NIST Handbook. 44 (2012 ed.). Washington, D.C.: U.S. Department of Commerce, Technology Administration, National Institute of Standards and Technology. ISSN 0271-4027. OCLC 58927093. Retrieved 6 July 2012.
- Rowlett, Russ (13 September 2001). "F". How Many? A Dictionary of Units of Measurement. Chapel Hill, NC: University of North Carolina at Chapel Hill. fluid dram or fluidram (fl dr). Retrieved 6 July 2012.
- Buchholz, Susan; Henke, Grace (2009). "Chapter 3: Metric, Apothecary, and Household Systems of Measurement — Table 3-1: Apothecary Abbreviations". Henke's Med-Math: Dosage Calculation, Preparation and Administration (6th ed.). Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams & Wilkins. p. 55. ISBN 978-0-7817-7628-8. OCLC 181600929. Retrieved 6 July 2012.
- Pickar, Gloria D.; Swart, Beth; Graham, Hope; Swedish, Margaret (2012). "Appendix B: Apothecary System of Measurement — Apothecary Units of Measurement and Equivalents". Dosage Calculations (2nd Canadian ed.). Toronto: Nelson Education. p. 528. ISBN 978-0-17-650259-1. OCLC 693657704. Retrieved 6 July 2012.
- United Kingdom; Department of Trade and Industry (1995). The Units of Measurement Regulations 1995. London: HMSO. Schedule: Relevant Imperial Units, Corresponding Metric Units and Metric Equivalents. ISBN 978-0-11-053334-6. OCLC 33237616. Retrieved 1 July 2012.
- 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.
- Great Britain (1878). Statutes at large .... p. 308. Retrieved 12 September 2012.
- Chisholm, Hugh (1911). The Encyclopædia Britannica: A Dictionary of Arts, Sciences, Literature and General Information. At the University Press. p. 480. Retrieved 12 September 2012.
- Great Britain (1855). A collection of public general statutes passed in the 18th and 19th years of the reign of Her Majesty Queen Victoria. pp. 273–75. Retrieved 5 January 2012.
- Weights and Measures Act
- 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..."
- The Council of the European Communities (27 May 2009). "Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC". Retrieved 13 September 2011.
- "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.
- "Department for Transport statement on metric road signs" (online). BWMA. 12 July 2002. Retrieved 24 August 2009.
- "In praise of ... metric measurements". The Guardian (London). 1 December 2006. Retrieved 5 May 2010.
- "Weights and Measures Act: Canadian units of measure". Justice Canada. Retrieved 14 November 2007.[dead link]
- "11". "Guide to Food Labelling and Advertising". Canadian Food Inspection Agency. Retrieved 1 December 2007.
- "Consumer Packaging and Labelling Regulations (C.R.C., c. 417)". Justice Canada, Legislative Services Branch. Retrieved 15 November 2012.
- "A Canadian compromise". CBC. Retrieved 12 March 2008.
- "A Canadian compromise". CBC. Retrieved 11 March 2008.
- "Les livres et les pieds, toujours présents (eng:The pounds and feet, always present)" (in French). 5 sur 5, Société Radio-Canada. Retrieved 11 March 2008.
- Britishweights And Measures Association
- [dead link]
- [dead link]
- [dead link]
- Home Hardware - Building Supplies - Building Materials - Fence Products
- Fuel Consumption Ratings | Office of Energy Efficiency
- Transportation Safety Board | Home
- "Metric usage and metrication in other countries". US Metric Association. Retrieved 2010-09-02. (Archive: 2 September 2010).
- "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)"
- "The High Commission Antigua and Barbuda". Retrieved 15 January 2008., "FuelPrices2005" (PDF). German Technical Cooperation. p. 96. Retrieved 15 January 2008.
- Sierra Leone Embassy to the United States INTRODUCTION OF THE METRIC SYSTEM AND THE PRICE OF PETROLEUM PRODUCTS Retrieved 23 October 2011.
- "Minister Lovell Addresses Metric Conversions". CARIBARENA Antigua. 18 October 2011. Retrieved 23 October 2011.
- 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.
- Appendices B and C of NIST Handbook 44
- Thompson, A.; Taylor, Barry N. (5 October 2010). "The NIST guide for the use of the international system of units". also available as a PDF file. NIST. Retrieved 15 October 2012.
- 6 George IV chapter 12, 1825 (statute)
- Fast, simple, easy to use and intuitive unit converter
- British Weights And Measures Association
- Canada Weights and Measures Act 1970-71-72[dead link]
- General table of units of measure – NIST – pdf
- How Many? A Dictionary of Units of Measurement
- Statutory Instrument 1995 No. 1804 Units of Measurement Regulations 1995