Imperial Measurement Units

What are Imperial Units?

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Imperial units are the measurement units that were historically used in the British Commonwealth countries.  They were very similar, but not identical, to the units that are still predominantly used in the United States.

The Commonwealth countries have since switched to the SI system of units.  Because references to the units of the old British customary imperial units are still found, the following discussion describes the differences between the U.S. and British customary systems.

Differences between the U.S. and British Customary Systems

Measures of length
After 1959, the U.S. and the British inch were defined identically for scientific work and were identical in commercial usage (however, the U.S. retained the slightly different survey inch for specialized surveying purposes). 

Measures of volume
The U.S. customary bushel and the U.S. gallon, and their subdivisions differ from the corresponding British Imperial units. 

Also, the British ton is 2240 pounds, whereas the ton generally used in the United States is the short ton of 2000 pounds.

The American colonists adopted the English wine gallon of 231 cubic inches.  The English of that period used this wine gallon and they also had another gallon, the ale gallon of 282 cubic inches.  In 1824, the British abandoned these two gallons when they adopted the British Imperial gallon, which they defined as the volume of 10 pounds of water, at a temperature of 62°F, which, by calculation, is equivalent to 277.42 cubic inches. At the same time, they redefined the bushel as 8 gallons.