Comparison of the imperial and US customary measurement systems
Both the imperial and United States customary systems of measurement derive from earlier English systems used in the Middle Ages, that were the result of a combination of the local Anglo-Saxon units inherited from German tribes and Roman units brought by William the Conqueror after the Norman Conquest of England in 1066.
Having this shared heritage, the two systems are quite similar, but there are differences. The US customary system is based on English systems of the 18th century, while the Imperial system was defined in 1824, after American independence.
Volume may be measured either in terms of units of cubic length or with specific volume units. The units of cubic length (the cubic inch, cubic foot, cubic mile, etc.) are the same in the imperial and US customary systems but with the specific units of volume (the bushel, gallon, fluid ounce, etc.) they differ. The US customary system has one set of units for fluids and another set for dry goods. The imperial system has only one set defined independently of and subdivided differently from its US counterparts.
By the end of the eighteenth century various systems of volume measurement were in use throughout the British Empire. Wine was measured with units based on the Queen Anne's gallon of 231 cubic inches (3.785 L). Beer was measured with units based on an ale gallon of 282 cubic inches (4.621 L). Grain was measured with the Winchester measure with a gallon of approximately 268.8 cubic inches (one eighth of a Winchester bushel or 4.405 L). In 1824 these were replaced with a single system based on the imperial gallon. Originally defined as the volume of 10 pounds (4.5 kg) of distilled water (under certain conditions), then redefined by the Weights and Measures Act 1985 to be exactly 4.54609 L (277.4 cu in), the imperial gallon is close in size to the old ale gallon.
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