In metrology, Imperial units (also called British units) are the measurement units that were generally used in the British Commonwealth countries in the past.

Table of contents
1 Relation to other systems
2 Measures of length
3 Measures of volume
4 Measures of weight and mass

Relation to other systems

Most Imperial units had the same names as to the units that are still predominantly used in the United States (see U.S. customary units). Unfortunately, the detailed definitions differed, and in some cases the differences are substantial. The Commonwealth countries have since switched to the SI system of units. In the United Kingdom, some imperial units are still retained (e.g. miles in road signs), but the use of metric (SI) units is increasingly mandated by law for the sale of food etc. Because references to the units of the old British customary imperial units are still found, the following discussion describes the differences between the U.S. and British customary systems.

A further difference between the systems in use in the two countries is that in cooking weights and measures, much more use is made of volume measures (cups and spoons) in the US, whereas in the UK quantities of dry ingredients are usually specified by weight; cup and spoon measurements are sometimes given, but these are not the same as the US standard cups and spoons, and in traditional recipes probably just reflect a favourite cup that the cook had to hand.

Measures of length

After 1959, the U.S. and the British inch were defined identically for scientific work and were identical in commercial usage (however, the U.S. retained the slightly different survey inch for specialized surveying purposes). The tables of length, such as 12 inches = 1 foot, 3 feet = 1 yard, and 1760 yards = 1 international mile, were the same in both countries, though some of the intermediate units such as the chain (22 yards) and the furlong (220 yards) were more used in Britain than in the U.S.

Measures of volume

The present British gallon and bushel--known as the "Imperial gallon" and "Imperial bushel"--are, respectively, about 20 percent and 3 percent larger than the United States gallon and bushel. The Imperial gallon is defined as the volume of 10 avoirdupois pounds of water under specified conditions, and the Imperial bushel is defined as 8 Imperial gallons. Also, the subdivision of the Imperial gallon as presented in the table of British apothecaries' fluid measure differs in two important respects from the corresponding United States subdivision, in that the Imperial gallon is divided into 160 fluid ounces (whereas the United States gallon is divided into 128 fluid ounces), and a "fluid scruple" is included.

The full table of British measures of capacity (which are used alike for liquid and for dry commodities) is as follows:

  • 4 gills = 1 pint
  • 2 pints = 1 quart
  • 4 quarts = 1 gallon
  • 2 gallons = 1 peck
  • 8 gallons (4 pecks) = 1 bushel
  • 8 bushels = 1 quarter

The full table of British apothecaries' measure is as follows:
  • 20 minims = 1 fluid scruple
  • 3 fluid scruples = 1 fluid drachm = 60 minims
  • 8 fluid drachms = 1 fluid ounce
  • 20 fluid ounces = 1 pint
  • 8 pints = 1 gallon (160 fluid ounces)

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 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, which, by calculation, is equivalent to 277.42 cubic inches - much closer to the ale gallon than the wine gallon. At the same time, they redefined the bushel as 8 gallons.

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 U.S. gallon is divided into four liquid quarts and the U.S. bushel into 32 dry quarts. All the units of capacity or volume mentioned thus far are larger in the customary British system than in the U.S. system. But the British fluid ounce is smaller than the U.S. fluid ounce, because the British quart is divided into 40 fluid ounces whereas the U.S. quart is divided into 32 fluid ounces.

From this we see that in the customary British 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 gallon is equivalent to 4 quarts, or 160 fluid ounces. This convenient relation does not exist in the U.S. system because a U.S. gallon of water at 62°F weighs about 8 1/3 pounds, or 133 1/3 avoirdupois ounces, and the U.S. gallon is equivalent to 4 x 32, or 128 fluid ounces.

  • 1 U.S. fluid ounce = 1.041 British fluid ounces
  • 1 British fluid ounce = 0.961 U.S. fluid ounce
  • 1 U.S. gallon = 0.833 British Imperial gallon
  • 1 British Imperial gallon = 1.201 U.S. gallons

In the apothecary system of liquid measure the British add a unit, the fluid scruple, equal to one third of a fluid drachm (spelled dram in the United States) between their minim and their fluid drachm.

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 was abolished in Britain on January 6, 1879, with only the troy ounce and its subdivisions retained, whereas the troy pound (of 12 troy ounces) is still legal in the United States, although it is not now greatly used. Another important difference is the universal use in Britain, for body weight, of the stone of 14 pounds, 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 avoirdupois pound, the troy pound, and the apothecaries' pound are identical in Britain and the United States. The tables of British 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 British avoirdupois mass is the same as the United States table up to 1 pound; above that point the table reads:

  • 14 pounds = 1 stone
  • 2 stones = 1 quarter = 28 pounds
  • 4 quarters = 1 hundredweight = 112 pounds
  • 20 hundredweight = 1 ton = 2240 pounds
Note that the British ton is 2240 pounds (known in the US as a "long ton", whereas the ton generally used in the United States is the "short ton" of 2000 pounds.

Based on Appendices B and C of NIST Handbook 44. (Being a U.S. Government publication, it is presumably public domain).