Comparison of the imperial and US customary measurement systems
From Wikipedia, the free encyclopedia
Both the imperial and United States customary systems of measurement derive from earlier English systems. These English systems had developed in England over several centuries since the Battle of Hastings in 1066. They were a combination of the Anglo-Saxon and Roman systems.
Having this shared heritage, the two systems are quite similar, however there exist differences between them. The US customary system is based on English systems of the 1700s, while the Imperial system was defined in 1824, which happened after American independence.
In the form of the US customary system, English units are still very much in use in the United States despite official adoption of the International System of Units (SI). In contrast, the use of these units in Commonwealth countries, where the introduction of the SI has been more successful, is limited and declining.
Contents |
[edit] Volume
The imperial gallon (4.546 09 litres (L)) and bushel (36.368 72 L) are, respectively, 20% and about 3% larger than the United States liquid gallon (3.785411784 L) and bushel (35.23907016688 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.
The origins of these differences lie in the variety of systems that were in use in Britain at the time of the establishment of the first colonies in North America. The American colonists adopted the English wine gallon of 231 cubic inches, and used it for all fluid purposes. The English of that period used this wine gallon, but they also had another gallon, the ale gallon of 282 cubic inches. In 1824, the British abandoned these two gallons when they adopted the British imperial gallon, which they defined as the volume of 10 pounds of water, at a temperature of 62 °F, weighed in air with brass weights, which, by calculation, is equivalent to about 277.42 cubic inches (4,546.1 cm³)—much closer to the ale gallon than the wine gallon. At the same time, they redefined the bushel to be 8 gallons.
Even under the new imperial system, wine volumes continued to be measured in the old 231 cubic inch wine gallons, for tax purposes, and this practice continued until the late 1990s.
As noted above, in the customary British system the units of dry measure are the same as those of liquid measure. In the United States these two are not the same, the gallon and its subdivisions are used in the measurement of liquids; the bushel, with its subdivisions, is used in the measurement of certain dry commodities. The US gallon (3.785 411 784 L) is divided into four liquid quarts (946.352 946 mL each) and the US bushel (35.239 070 166 88 L) into 32 dry quarts (1.101 220 942 715 L each) or 4 pecks (8.809 767 541 72 L each). All the units of volume mentioned thus far are larger in the imperial system than in the US system (by about 20% for wet measures). But the British fluid ounce is smaller than (about 96% of) the US fluid ounce, because the British quart is divided into 40 fluid ounces whereas the US quart is divided into 32 fluid ounces.
From this we see that, in the imperial system, an avoirdupois ounce of water at 62 °F has a volume of one fluid ounce, because 10 pounds is equivalent to 160 avoirdupois ounces, and 1 imperial gallon is equivalent to 4 imperial quarts, or 8 pints. This convenient fluid-ounce-to-avoirdupois-ounce relation does not exist in the US system because a US gallon of water at 62 °F weighs about 81⁄3 pounds, or 1331⁄3 avoirdupois ounces, and the US gallon is equivalent to 4 × 32, or 128 fluid ounces.
| Imperial measure | US measure | metric measure |
|---|---|---|
|
= 1 Imperial fluid ounce |
≈ 0.960 759 9 US fluid ounce |
= 28.413 062 5 millilitres |
|
≈ 1.040 842 7 Imperial fluid ounces |
= 1 US fluid ounce |
= 29.573 529 562 5 millilitres |
|
≈ 35.195 080 Imperial fluid ounces |
≈ 33.814 023 US fluid ounces |
= 1 litre |
|
≈ 133.227 87 Imperial fluid ounces |
= 128 US fluid ounces |
= 3.785 411 784 litres |
|
≈ 155.028 79 Imperial fluid ounces |
≈ 148,945 45 US fluid ounces |
= 4.404 842 803 2 litres |
|
= 160 Imperial fluid ounces |
≈ 153.721 59 US fluid ounces |
= 4.546 09 litres |
In the apothecary system of liquid measure the British add a unit, the fluid scruple, equal to one third of a fluid drachm (spelt dram in the United States) between their minim and their fluid drachm.
One noticeable comparison between the imperial system and the U.S. Customary system are between some Canadian beer bottles and American beer bottles. Many Canadian brewers, like Labatt, package beer in 12 imperial fluid ounce bottles, which are 341 mL each. American brewers package their beer in 12 U.S. fluid ounce bottle, which are 355 mL each. This results in the Candaian bottles being labeled as 11.5 fl oz in U.S. units when imported into the United States.
[edit] Length
The international yard is defined in terms of the metric system of units to be exactly 0.9144 metres. This definition was agreed upon by the US, Canada, the UK, South Africa, Australia and New Zealand in 1959. However, the US continued to use its previous national definition of the length units for surveying purposes.
The US survey foot is defined so that 1 metre is exactly 39.37 inches; the international foot is exactly two parts per million shorter. The U.S. survey foot and survey mile have been maintained as separate units for surveying purposes. This was done to avoid the accumulation of error it would entail replacing them with the international versions. This was not a problem for the United Kingdom, as the Ordnance Survey has been metric since before World War II.
The main units of length (inch, foot, yard and international mile) were the same in the USA, though some of the intermediate units such as the (surveyor's) chain (22 yards) and the furlong (220 yards) were hardly used there.
At one time the nautical mile was defined differently in the UK and the US however today both countries use the international definition of 1852 metres. For more details refer to the Nautical mile article.
[edit] Weight
A discussion of differences between countries is complicated by the fact that both Britain and the US 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, virtually unused ever since the metric system has been in use for all scientific purposes.
Among other differences between the customary British and the United States measurement systems, 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 widespread use in Britain, for body weight, of the stone of 14 pounds (6.350 293 18 kg), a unit unused in the United States, although its influence was seen in the practice until World War II of selling flour by a barrel of 196 pounds (14 stone).
In all the systems, the fundamental unit is the pound (lb), 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, but above that point the tables differ.
The imperial system uses a hundredweight of eight stone or 112 lb (50.802 345 44 kg), whereas a US hundredweight is 100 lb (45.359 237 kg). In both systems, 20 hundredweights make a ton. In the US, the terms long ton (2,240 lb, 1016.046 908 8 kg) and short ton (2,000 lb; 907.184 74 kg) are used to distinguish them. The term metric ton is also used to denote a tonne (1,000 kg, 2,204.622 lb), which is coincidentally within less than 2% of the long ton.
