Imperial gallon is legally defined as 4.54609 litres. This definition is used in Commonwealth countries and Ireland, and is based on the volume of 10 pounds of water at 62 °F. A U.S. liquid gallon of water weighs about 8.33 pounds at the same temperature. The Imperial fluid ounce is defined as 1/160 of an Imperial gallon.
Imperial units or the Imperial system is a collection of units, first defined in the British Weights and Measures Act of 1824, later refined (until 1959) and reduced. Systems of imperial units are sometimes referred to as foot-pound-second, after the base units of length, mass and time. The units were introduced in the British Empire, excluding the then already independent United States. As of 2008, all countries that used the Imperial system have become officially metric (except for Myanmar), but Imperial units continue to be used alongside metric units.
The distinction between the imperial system and the U.S. customary units (also called standard or English units) or older British/English units/systems and newer additions is often not drawn precisely. Most length units are shared between the imperial and U.S. systems, albeit partially and temporally defined differently. Capacity measures differ the most due to the introduction of the imperial gallon and the unification of wet and dry measures. The avoirdupois system applies only to weights; it has a long designation and a short designation for the hundredweight and ton. The term imperial should not be applied to English units that were outlawed in Weights and Measures Act of 1824 or earlier, or which had fallen out of use by that time, nor to post-imperial inventions such as the slug or poundal.
Although most of the units are defined in more than one system, some subsidiary units were used to a much greater extent, or for different purposes, in one area rather than the other.