Metric system

The metric system is a decimal-based system of measurement. The current international standard for the metric system is the International System of Units (Système international d'unités or SI), in which all units can be expressed in terms of seven base units: the metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (Cd). These can be made into larger or smaller units with the use of metric prefixes.

SI derived units are named combinations – such as the hertz (cycles per second), newton (kg⋅m/s²), and tesla (1 kg⋅s⁻²⋅A⁻¹) – or a shifted scale, in the case of degrees Celsius. Certain units have been officially accepted for use with the SI. Some of these are decimalized, like the litre and electronvolt, and are considered "metric". Others, like the astronomical unit are not. Ancient non-metric but SI-accepted multiples of time (minute and hour) and angle (degree, arcminute, and arcsecond) are sexagesimal (base 60).

The "metric system" has been formulated in a number of different ways over the centuries. The SI system originally derived its terminology from the metre, kilogram, second system of units, though the definitions of the fundamental SI units have been changed to depend only on constants of nature. Other metric system variants include the centimetre–gram–second system of units, the metre–tonne–second system of units, and the gravitational metric system. Each of these has some unique named units (in addition to unaffiliated metric units) and some are still in use in certain fields.