A measurement system is a collection of units of measure and related rules. Measurement systems have historical significance and are regulated and defined for the purposes of science and commerce. Measurement systems in use include the International System of Units or SI (the modern form of the metric system), the British Imperial System, and the traditional system of the United States.

Measurement is comprehensive or nearly comprehensive in almost all countries. Units of the traditional US system are widely used in the United States and to some extent in Liberia. Traditional measurement units from Myanmar are used in Myanmar. The US units are used in limited contexts in Canada due to the high volume of trade; there is also significant use of imperial weights and measures, despite Canada's legal adjustment to the metric system. A number of other jurisdictions have laws that enforce or allow other measurement systems in certain or all contexts, such as the United Kingdom - whose legislation on road signage only allows distance signs that display imperial units (miles or yards) - or Hong Kong.

In the United States, metric units are nearly universally used in science, widely in military service, and partially in industry, but traditional units dominate household use. In stores, liters are a commonly used unit for volume, especially in beverage bottles, and milligrams, instead of grains, are used for medications. Some other standardized non-SI units are still in international use, such as miles and knots in aviation and shipping.