A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce. Systems of measurement in use include the International System of Units or SI (the modern form of the metric system), the British imperial system, and the United States customary system.

Metrication is complete or nearly complete in almost all countries. US customary units are heavily used in the United States and to some degree in Liberia. Traditional Burmese units of measurement are used in Burma. U.S. units are used in limited contexts in Canada due to the large volume of trade; there is also considerable use of Imperial weights and measures, despite the jure Canadian conversion to metric. A number of other jurisdictions have laws mandating or permitting other systems of measurement in some or all contexts, such as the United Kingdom - whose road signage legislation, for instance, only allows distance signs displaying imperial units (miles or yards) - or Hong Kong.

In the United States, metric units are used almost universally in science, widely in the military, and partially in industry, but customary units predominate in household use. At retail stores, the litter is a commonly used unit for volume, especially on bottles of beverages, and milligrams, rather than grains, are used for medications. Some other standard non-Si units are still in international use, such as miles and knots in aviation and shipping.