Comparison of the imperial and US customary measurement systems adopted a system based on the 231-cubic-inch wine gallon for all fluid purposes. This became the US fluid gallon. Both the imperial and US fluid gallon are divided into 4 quarts, 8 pints or 32 gills. However, whereas the US gill is divided into 4 US fluid ounces, the imperial gill is divided into 5 imperial fluid ounces. So whilst the imperial gallon, quart, pint and gill are about 20% larger than their US fluid measure counterparts, the fluid ounce is about 4% smaller. Note that one avoirdupois ounce of water has an approximate volume of one imperial fluid ounce
Gallon one time defined as the volume of a cylinder 6 inches deep and 7 inches in diameter, i.e. . It had been redefined during the reign of Queen Anne, in 1706, as 231 cubic inches exactly, which is the result of the earlier definition with π approximated to . Although the wine gallon had been used for centuries for import duty purposes there was no legal standard of it in the Exchequer and a smaller gallon was actually in use, so this statute became necessary. It remains the US definition today. In 1824, Britain adopted a close approximation to the