Imperial units
This article is about post-1824 Imperial units, please see also English unit, U.S. customary unit or Avoirdupois.
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The Imperial units are 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, but excluding the then already independent United States.
Relation to other systems
The distinction between this imperial system and the U.S. customary units (also called standard units there) or older British/English units/systems and newer additions (foot-pound-second systems) 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; the Imperial system only uses its long flavour, the U.S. customary system uses the short flavour.
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
After the 1 July 1959 deadline, agreed upon in 1958, the U.S. and the British yard were defined identically (0.9144 m), the international yard. Given metric equivalents in this article usually assume this latest official definition.
| 1 inch | = 25.4 mm | |||
|---|---|---|---|---|
| 1 foot | = 12 inches | = 304.8 mm | ||
| 1 yard | = 3 feet | = 36 inches | = 914.4 mm | |
| 1 rod, pole or perch | = 5½ yards | = 16½ feet | = 5.0292 m | |
| 1 chain | = 4 poles | = 22 yards | = 66 feet | = 20.1168 m |
| 1 furlong | = 10 chains | = 220 yards | = 660 feet | = 201.168 m |
| 1 mile | = 8 furlongs | = 1760 yards | = 5280 feet | = 1.609 344 km |
| 1 league | = 3 miles | = 5280 yards | = 15840 feet | = 4.828 032 km |
However, the nautical mile equalled 6080 feet (1.85318 km), contrary to 6080.2 ft (1.853249 km) in the USA, 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
| 1 rood | = 1 furlong × 1 rod | = 40 square rods | = 10890 square feet | = 0.10117141056 ha | = 1011.7141056 m² |
|---|---|---|---|---|---|
| 1 acre | = 1 furlong × 1 chain | = 160 square rods | = 1/640 square mile | = 0.40468564224 ha | = 4046.8564224 m² |
Measures of volume
The Imperial gallon (4.546 09 L) and bushel (36.368 72 L) are, respectively, about 20 per cent and 3 per cent larger than the United States liquid gallon (3.785 411 784 L) and bushel (35.239 070 166 88 L). The Imperial gallon was originally defined as the volume of 10 avoirdupois pounds of water under specified conditions, and the Imperial bushel was defined as 8 Imperial gallons. Also, the subdivision of the Imperial gallon as presented in the table of British apothecaries' fluid measure differed in two important respects from the corresponding United States subdivision, in that the Imperial gallon was divided into 160 fluid ounces (whereas the United States gallon is divided into 128 fluid ounces), and a "fluid scruple" is included.
| 1 fluid ounce | = 0.028 413 062 5 L | |||
|---|---|---|---|---|
| 1 gill | = 5 oz. | = 0.142 065 312 5 L | ||
| 1 pint | = 4 gills | = 20 oz. | = 0.568 261 25 L | |
| 1 quart | = 2 pints | = 40 oz. | = 1.136 522 5 L | |
| 1 gallon | = 4 quarts | = 8 pints | = 160 oz. | = 4.546 09 L |
| 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 |
| 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 (of beer) | = 3 kilderkin | = 6 firkin | = 54 gal. | = 245.488 86 L |
The full table of British apothecaries' measure is as follows:
| 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 fluid scruples | = 60 minims | = 3.551 632 812 5 mL |
| 1 fluid ounce | = 8 fluid drachms | = 480 minims | = 28.413 062 5 mL |
| 1 pint | = 20 fluid ounces | = 568.261 25 mL | |
| 1 gallon | = 8 pints | = 160 fluid ounces | = 4.546 09 L |
For a comparison to the U.S. customary system see the article on English units
Measures of weight and mass
A discussion of differences between countries is complicated by the fact that both Britain and the U.S. have made some use of three different weight systems, 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.
Among other differences between the customary British and the United States measurement systems, we should note that 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, whereas the troy pound (of 12 troy ounces) and pennyweight are still legal in the United States, although they are not now greatly used. Another important difference is the universal use in Britain, for body weight, of the stone of 14 pounds (6.350 293 18 kg), this being a unit now unused in the United States, although its influence was shown in the practice until World War II of selling flour by the barrel of 196 pounds (14 stone).
In all the systems, the fundamental unit is the pound, and all other units are defined as fractions or multiples of it. The tables of Imperial troy mass and apothecaries' mass are the same as the corresponding United States tables, except for the British spelling "drachm" in the table of apothecaries' mass. The table of Imperial avoirdupois mass is the same as the United States table up to 1 pound, above that point the table differs:
| 1 mite | = 1/20 grain | = 3.239 945 5 mg | |
|---|---|---|---|
| 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 stones | = 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.
British law now defines each Imperial unit entirely in terms of the metric equivalent. See the Units of Measurement Regulations 1995. This regulation effectively outlaws their usage in retail and trading except in previously established exceptions. This has now been proved by in court against the so called 'Metric Martyrs', a small group of market traders. Despite this, many small market traders still use the customary measures, citing customer preference especially among the older population.
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.
All kinds of English units have been replaced elsewhere by the SI (metric) system. Most Commonwealth countries have switched entirely to the international system of units.
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 have miles, 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 use of SI units is increasingly 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 yards), speed (miles per hour), weight (stones and pounds), liquid (pints and gallons) and height (feet and inches).
In Canada, the government's efforts to implement the metric system were more extensive: pretty much any agency, institution, or thing provided by the government will use 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-storey parking facility). Temperatures in degrees Fahrenheit will occasionally be heard on English Canadian commercial radio stations, but only those that cater to older listeners. 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. Traditional units persist in ordinary conversation and may be experiencing a resurgence due to the reduction in trade barriers with the United States. 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, of course), 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 (although "two-by-fours" don't actually measure 2×4", for example).
See also
References
- Appendices B and C of NIST Handbook 44
- Barry N. Taylor's NIST Special Publication 811, also available as a PDF file