Systems of Measurement

SI, also known as the metric system. The International System of Units (abbreviated SI from the French phrase, Système International d'Unités) is the most widely used system of units. It is used for everyday commerce in virtually every country of the world except the United States, Liberia and Myanmar, and it is almost globally used in scientific and engineering work. In 1960, SI was selected as a specific subset of the existing Metre-Kilogram-Second systems of units (MKS), rather than the older Centimetre-Gram-Second system of units (CGS). Various new units were added with the introduction of the SI and at later times. SI is sometimes referred to as the metric system (especially in the United States, which has not widely adopted it, although it has been used more commonly in recent years, and in the UK, where conversion is incomplete). The International System of Units refers to a specific canon of measurements derived and extended from the Metric system; however, not all metric units of measurement are accepted as SI units.

U.S. Customary Units, commonly known in the United States as English units or standard units, are the non-metric units of measurement that are presently used in the U.S., in some cases alongside the metric system of units. This system of units is similar to the Imperial system (sometimes called the British system) once used in the United Kingdom and the Commonwealth, but has some differences. Both systems derive from the evolution of local units over the centuries as a result of standardisation efforts in England; the local units themselves mostly trace back to Roman units. Today, U.S. customary units are defined in terms of SI units.