Imperial units

This article is about post-1824 Imperial units, please see also English unit, U.S. customary unit or Avoirdupois.

The Imperial units or the Imperial system is a collection of English units, first defined in the Weights and Measures Act of 1824, later refined (until 1959) and reduced. The units were introduced in the United Kingdom and its colonies, including Commonwealth countries (though most Commonwealth countries are now fully metricated), but excluding the then already independent United States. Systems of Imperial units are sometimes referred to as foot-pound-second, after the base units of length, mass and time.

The former Weights and Measures office in Middlesex, England.

Relation to other systems

The distinction between this imperial system and the U.S. customary units (also called standard or English units there) or older British/English units/systems and newer additions is often not drawn precisely. Most length units are shared among the Imperial and U.S. systems, albeit partially and temporally defined slightly 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 only applies to weights; it has a long flavour and a short flavour for the hundredweight and ton.

The term imperial should not be applied to English units that were outlawed in Weights and Measures Act of 1824 or earlier, or which had fallen out of use by that time, nor to post-imperial inventions such as the slug or poundal.

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.

Measures of length

Imperial standards of length 1876 in Trafalgar Square, London.

After the 1 July 1959 deadline, agreed upon in 1958, the U.S. and the British yard were defined identically (0.9144 m) to 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 m (Sears et al. 1928. Phil Trans A 227:281).

Length

	inch	link	foot	yard	pole	chain	furlong	mile	league	metre
1 inch	1	25⁄198	1⁄12	1⁄36	1⁄198	1⁄792	1⁄7,920	1⁄63,360	1⁄190,080	0.0254
1 link	7+23⁄25	1	33⁄50	11⁄50	1⁄25	1⁄100	1⁄1,000	1⁄8,000	1⁄24,000	0.201168
1 foot	12	1+17⁄33	1	1⁄3	2⁄33	1⁄66	1⁄660	1⁄5,280	1⁄15,840	0.3048
1 yard	36	4+6⁄11	3	1	2⁄11	1⁄22	1⁄220	1⁄1,760	1⁄5,280	0.9144
1 pole	198	25	16+1⁄2	5+1⁄2	1	1⁄4	1⁄40	1⁄320	1⁄960	5.0292
1 chain	792	100	66	22	4	1	1⁄10	1⁄80	1⁄240	20.1168
1 furlong	7,920	1,000	660	220	40	10	1	1⁄8	1⁄24	201.168
1 mile	63,360	8,000	5,280	1,760	320	80	8	1	1⁄3	1,609.344
1 league	190,080	24,000	15,840	5,280	960	240	24	3	1	4,828.032

Until the adoption of the international definition of 1852 metres in 1970, the British nautical mile was defined as 6080 feet (1.85318 km). 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). Depth of water at sea was expressed in fathoms (6 feet = 1.8288 m).

Measures of area

Area

1 rood	= 1 furlong × 1 rod	= 40 square rods	= 1⁄2560 square mile	= 10890 square feet	= 0.10117141056 ha	= 1011.7141056 m²
1 acre	= 1 furlong × 1 chain	= 160 square rods	= 1⁄640 square mile	= 43560 square feet	= 0.40468564224 ha	= 4046.8564224 m²

Measures of volume

In 1824, Britain 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 inHg and at a temperature of 62 °F. In 1963, this definition was refined as the space occupied by 10 lb of distilled water of density 0.998 859 g/ml weighed in air of density 0.001 217 g/ml against weights of density 8.136 g/ml. This works out to exactly 4.545 964 591 l, or 277.420 in³. The Weights and Measures Act of 1985 finally switched to a gallon of exactly 4.546 09 l (approximately 277.419 43 cu in).

Wet

	ounce	gill	pint	quart	gallon	litre
1 (fluid) ounce	1	1⁄5	1⁄20	1⁄40	1⁄160	0.028 413
1 gill	5	1	1⁄4	1⁄8	1⁄32	0.142 065
1 pint	20	4	1	1⁄2	1⁄8	0.568 261
1 quart	40	8	2	1	1⁄4	1.136 522
1 gallon	160	32	8	4	1	4.546 09

Dry

1 peck			= 2 gal.	= 9.092 18 l
1 kenning or bucket		= 2 pecks	= 4 gal.	= 18.184 36 l
1 bushel	= 2 kennings	= 4 pecks	= 8 gal.	= 36.368 72 l
1 strike		= 2 bushels	= 16 gal.	= 72.737 44 l
1 quarter or pail		= 8 bushels	= 64 gal.	= 290.949 76 l
1 chaldron	= 4 quarters	= 32 bushels	= 256 gal.	= 1163.799 04 l
1 last	= 10 quarters	= 80 bushels	= 640 gal.	= 2909.497 6 l

Brewery

1 firkin			= 9 gal.	= 40.914 81 l
1 kilderkin		= 2 firkin	= 18 gal.	= 81.829 62 l
1 barrel	= 2 kilderkin	= 4 firkin	= 36 gal.	= 163.659 24 l
1 hogshead	= 3 kilderkin	= 6 firkin	= 54 gal.	= 245.488 86 l

The full table of British apothecaries’ measure is:

Apothecaries’ measure

1 minim			= 0.059 193 880 208¯3 ml
1 (fluid) scruple		= 20 minims	= 1.183 877 604 1¯6 ml
1 (fluid) dram or fluidram	= 3 scruples	= 60 minims	= 3.551 632 812 5 ml
1 fluid ounce	= 8 drachms	= 480 minims	= 28.413 062 5 ml
1 pint		= 20 ounces	= 568.261 25 ml
1 gallon	= 8 pints	= 160 ounces	= 4.546 09 l

For a comparison to the U.S. customary system see the article on Comparison of the Imperial and US customary systems.

Measures of weight and mass

Britain has made some use of three different weight systems in the 19th and 20th century, 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 use of the troy pound (373.241 721 6 g) was abolished in Britain on January 6, 1879, with only the troy ounce (31.103 476 8 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.

Avoirdupois mass

1 grain			= 64.798 91 mg
1 drachm	= 1/16 ounce	= 1/256 pound	= 1.771 845 195 312 5 g
1 ounce		= 1/16 pound	= 28.349 523 125 g
1 pound	= 7000 grains		= 453.592 37 g
1 stone		= 14 pounds	= 6.350 293 18 kg
1 quarter	= 2 stone	= 28 pounds	= 12.700 586 36 kg
1 hundredweight	= 4 quarters	= 112 pounds	= 50.802 345 44 kg
1 ton	= 20 hundredweight	= 2240 pounds	= 1016.046 908 8 kg

Note that the British ton is 2240 pounds (the long ton), which is very close to a metric tonne, whereas the ton generally used in the United States is the "short ton" of 2000 pounds (907.184 74 kg), both are 20 hundredweights.

Further information: Comparison of the Imperial and US customary systems

Current use of Imperial units

In the United States and in a few Caribbean countries, the U.S. customary units, which are similar to Imperial units based upon older English units and in part share definitions, are still in common use. English units have been replaced elsewhere by the SI (metric) system. Most Commonwealth countries have switched entirely to the international system of units.

United Kingdom

Main article: Metrication in the United Kingdom

British law now defines each Imperial unit entirely in terms of the metric equivalent.

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 proved in court against the so called 'Metric Martyrs', a small group of market traders who insisted on trading only in Imperial units. Contrary to the impression given by some press reports, these regulations have never placed 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 retail trade.

The United Kingdom completed its legal transition to SI units in 1995, but a few such units are still in official use: draught beer must still be sold in pints, most roadsign distances are still in yards and miles, and speed limits are in miles per hour, therefore interfaces in cars must be capable of displaying miles per hour, and even though the troy pound was outlawed in Great Britain 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 also the Channel Tunnel Rail link high speed line, and around Ashford International Station and Dollands Moor International Freight Terminal, 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, but most British people still use Imperial units in colloquial discussion of distance (miles) and speed (miles per hour). This has also led to some slightly strange measures of common items; 568 millilitres containers of milk or 454 gramme bags of flour are commonly seen, being 1 pint and 1 pound respectively. Many 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 by the Police. For example 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 quoted in Miles per Gallon without exception. Fahrenheit equivalents are often given after the Celsius in weather forecasts.

Canada

Main article: Metrication in Canada

In Canada, the government's efforts to implement the metric system were more extensive: almost any agency, institution, or function provided by the government will use SI units exclusively. In the 1970s, the metric systems and SI units were introduced. 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). Temperature is typically measured and reported in degrees Celsius. Some radio stations near the border however (such as CIMX and CIDR), mainly or only use imperial units to report the weather. In addition, Environment Canada still has reports with an Imperial units option besides the metric ones. The law requires that measured products (such as fuel and meat) be priced in metric units, although there is leniency in regards to fruits and vegetables. Nevertheless, traditional units persist in ordinary conversation. Few Canadians would use SI units to describe their weight and height, although driver's 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. Land is surveyed and registered in metric units, but imperial units still dominate in construction, house renovation and gardening talk.

One area where Imperial units are still in common use is in firearms and ammunition. For example, Imperial measures are still used in the description of cartridge types, even where the cartridge is of relatively recent invention (e.g. .204 Ruger, .17 HMR, where the caliber is expressed in decimal fractions of an inch). However ammunition which is classified in metric already is still kept metric (e.g. 9 mm, 7.62 mm). In the manufacture of ammunition, bullet and powder weights are still expressed in terms of Imperial grains.

See also

• Conversion of units
• Cooking weights and measures
• Metrication
• Metric system
• Unit of measurement
• Systems of measurement
• History of measurement
• Board foot
• Cord (unit of volume)

References

• Appendices B and C of NIST Handbook 44
• Barry N. Taylor's NIST Special Publication 811, also available as a PDF file
• 6 George IV chapter 12, 1825 (statute)

External links

• Canada - Weights and Measures Act 1970-71-72
• British Weights And Measures Association
• Jacques J. Proot's Anglo-Saxon weights & measures page.
• General table of units of measure from the NIST