A measurement system, also a system of units, is a consistent system of measurement units in which to express physical quantities.

The most commonly used scientific measurement system is the SI system, the Système International d'Unités, which is based on the fundamental units meter (m) for length, kilogram (kg) for mass, and second (s) for time.

All other units (with few exceptions) are based on these fundamental units, e.g., velocity is measured in the unit meters per second, or m/s. Units can be enlarged or reduced according to need: e.g., the unit of electrostatic capacity is the farad, named after Michael Faraday, but typical capacitors are measured in nanofarads (nF) or even picofarads (pF). Already the kilogram is such a scaled unit: a thousand grams.

Another metric system is the centimetre-gram-second system CGS or cgs system which is based on the fundamental units centimetre (cm) for length, gram (g) for mass, and second (s) for time.

The Imperial system is based on the fundamental units of foot (ft) for length, pound (lb) for mass, and second (s) for time. The Imperial system is widely used for certain purposes (e.g., miles for distance, miles/hour for speed, pounds for the weight of food stuffs) in the United States, United Kingdom, India, Canada and several other countries.

The science which studies the operational realization of measurement units is called metrology.