Comparison of the imperial and US customary measurement systems

are the same in the imperial and US customary systems, but they differ in their specific units of volume (the bushel, gallon, fluid ounce, etc.).

The imperial system has only one set defined independently of, and subdivided differently from, its US counterparts.

In 1824, these units were replaced with a single system based on the imperial gallon.

[a] Originally defined as the volume of 10 pounds (4.54 kg) of distilled water (under certain conditions),[b] 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.

As with US dry measures, the imperial system divides the bushel into 4 pecks, 8 gallons, 32 quarts or 64 pints.

The American colonists adopted a system based on the 231-cubic-inch wine gallon for all fluid purposes.

Both the imperial and US fluid gallon are divided into 4 quarts, 8 pints or 32 gills.

This definition was approved by the United States, Canada, the United Kingdom, South Africa, Australia and New Zealand through the international yard and pound agreement of 1959, and corresponds with the previous 1930s British and American definitions of 1 inch being 25.4 mm.

The choice of unit for surveying purposes is based on the unit used when the overall framework or geodetic datum for the region was established; for example, much of the former British empire still uses the Clarke foot[further explanation needed] for surveying.

According to the National Institute of Standards and Technology, the survey foot is obsolete as of 1 January 2023, and its use discouraged.

At one time, the definition of the nautical mile was based on the surface area of the Clarke ellipsoid.

[further explanation needed] While the US used the full value of 1853.256 metres, in the British Commonwealth, this was rounded to 6080 feet (1853.184 m).

One important difference is the widespread use in Britain of the stone of 14 pounds (6.35029318 kg) for body weight; this unit is not used in the United States, although flour was sold by a barrel of 196 pounds (14 stone) until World War II.