gallon

(gal)

   1. a traditional unit of liquid volume, derived from the Roman galeta, which originally meant a pailful. Gallons of various sizes have been used in Europe ever since Roman times. In the United States, the liquid gallon is legally defined as exactly 231 cubic inches; this is equal to the old English wine gallon, which originated in medieval times but was not standardized until 1707, during the reign of Queen Anne. Some scholars believe the wine gallon was originally designed to hold 8 troy pounds of wine. The U. S. gallon holds 4 liquid quarts or exactly 3.785 411 784 liters; a U.S. gallon of water weighs about 8.33 pounds. American colonists were also familiar with the Elizabethan beer and ale gallon, which held 282 cubic inches (4.621 liters).

   2. a historic British unit of dry volume still used implicitly in the U.S. In the U.S., the term "gallon" is not used in dry measure, but if it were it would be equal to 1/2 peck, or 4 dry quarts, or 268.8025 cubic inches, or approximately 4.404 884 liters. This unit is the English corn or grain gallon, standardized during the reign of Elizabeth I in the sixteenth century. The earliest official definition of a dry gallon in Britain is a 1303 proclamation of Edward I, where the gallon is defined as the volume of 8 pounds of wheat; the current U.S. "gallon" contains about 7.5 pounds of wheat. Grain gallons have tended to be larger than liquid gallons throughout the history of British units, apparently because they were based on heaped rather than "struck" (leveled) containers. A container in which grain has been heaped above the top will hold as much as 25% more grain, and the traditional corn gallon is in fact 16.4% larger than the wine gallon.

   3. a British Imperial unit of volume larger than either of the American gallons. The Imperial Weights and Measures Act of 1824 established a new unit for all volumes, liquid or dry, replacing all the other gallons in previous use in Britain. The Imperial gallon, designed to contain exactly 10 pounds of distilled water under precisely defined conditions, holds exactly 4.546 09 liters or approximately 277.4194 cubic inches. The Imperial gallon equals 1.20095 U.S. liquid gallons (British wine gallons) or 1.03206 U.S. dry gallons (British corn gallons).

   4. a traditional unit of volume in Scotland equal to 4 Scots quarts. This is almost exactly 3 British Imperial gallons, 3.6 U.S. liquid gallons, or 13.63 liters.