Imperial and US customary systems

Imperial and US customary systems are related collections of measurement units used chiefly in the United Kingdom and United States. They share many names but differ in some definitions, notably for volume.

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International yard and pound: The international yard and pound are units defined exactly in terms of the metre and kilogram. These agreements establish the shared modern definitions of the yard and pound in both systems.
Imperial units: Imperial units are units defined under the British Weights and Measures Act of 1824 and subsequent legislation. This is the British system whose units are often mistaken for US customary units.
Weights and Measures Act 1824: The Weights and Measures Act 1824 established a system of Imperial weights and measures in Britain. It formalized the British system after American independence had already separated US measures.
United States customary units in manufacturing: United States customary units in manufacturing are inch-pound measures used in American industrial design and production. Manufacturing standards show how customary units persist in technical practice.
Metric system: The metric system is a decimal system of measurement based on units such as the metre, kilogram, and litre. Its decimal structure contrasts with the customary systems’ inherited unit relationships.
United States customary units: United States customary units are measurement units used in the United States, derived largely from earlier English units. This is the American system that diverges from Imperial measures in selected definitions.
English units: English units are measurement units that developed in England and influenced later British and American systems. They are the common historical source of many units retained on both sides of the Atlantic.
British road signs: British road signs are traffic signs used on public roads in the United Kingdom. Road distances and speed limits preserve visible uses of miles and miles per hour.
Unit of measurement: A unit of measurement is a defined quantity used as a standard for expressing measurements. Both systems organize named units around agreed standards.
US liquid gallon: The US liquid gallon is a volume unit equal to four US quarts or 231 cubic inches. It is smaller than the Imperial gallon despite sharing its name.