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 8+1⁄3 pounds, or 133+1⁄3 avoirdupois ounces, and the US gallon is equivalent to 4 × 32, or 128 fluid ounces.
