Units of capacity are fundamentally the same as units of volume, and can be defined in terms of such. In fact, the current definition of the United States gallon is as 231 cubic inches. Historically, however, they have often been defined in terms of the weight of some particular substance. For example, the British Imperial gallon was defined as the volume of 10 pounds of water at a temperature of 62 ° F.
The traditional British units of capacity varied in value depending on the substance being measured; thus, an ale gallon was different from a wine gallon. In most cases, the pint was defined to weigh 1 pound when containing the substance in question, making the gallon weigh 8 pounds. However, since the pint contained 20 fluid ounces while the pound contained 16 (or 12, in the case of some pounds that have been used) ounces of weight, this meant that the fluid ounce did not weigh 1 ounce. The 1824 Weights and Measures Act in the United Kingdom made the Imperial gallon (of water, at the 62° F standard temperature) weigh 10 pounds, which in turn made the pint weigh 1¼ pounds, and this made the fluid ounce weigh 1 ounce.
By contrast, the United States customary system, though derived from the traditional British system, kept both the concept of a pint weighing 1 pound and a fluid ounces weighing 1 ounces, by making the pint contain only 16 fluid ounces. The United States adopted two of the various sets of traditional measurements from Britain: the wine gallon became the standard gallon, used for measuring all liquids, while the bushel, rather than the gallon, was the standard for measuring dry commodities, and a bushel appropriate for wheat or barley became the standard for all dry commodities. The choice of these two standards meant that the liquid pint was different from the dry pint, unlike the British Imperial pint, which was identical for all commodities.
Because the traditional measures listed in the table below did not have constant values, but different ones were used for different commodities, no equivalents are given for these traditional measurement units. However, the corresponding tables for the British Imperial, United States customary liquid, and United States customary dry units of capacity have the values that were adopted for those three systems derived from traditional British units.
|Name of unit||Value in terms of smaller units|
|fluid drachm (dram)||60 minims|
|fluid ounce||8 fluid drachms|
|gill||5 fluid ounces|
|bag or sack||3 bushels|
|chaldron||12 bags (sacks)|
|wey||1 1/9 chaldrons|