Metric system

The metric system is a family of measurement systems that use a standard set of base units and decimal-based prefixes—like *kilo* and *milli*—to describe larger and smaller quantities. The most well-known example is the International System of Units (SI), but other systems, such as Gaussian units for electromagnetism, are also widely accepted in specialized fields.

The modern SI defines seven base units: the metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (cd). From these, derived units are formed by combining base units—for instance, the hertz (cycles per second), newton (kg·m/s²), and tesla (1 kg·s⁻²·A⁻¹). The degree Celsius is a shifted scale based on the kelvin, where 0 °C equals 273.15 K, and a change of 1 °C is the same as a change of 1 K.

SI evolved from the older metre-kilogram-second (MKS) system, but the definitions of its base units have changed over time. Today, every base unit is defined by physical constants rather than by physical prototypes. Other metric variants include the centimetre–gram–second (CGS) system, the metre–tonne–second (MTS) system, and the gravitational metric system. Some of these are still used in limited contexts, and non-SI units like the litre remain common.

Almost all countries have adopted SI as their official system of weights and measures. A notable exception is the United States, which uses the system in some contexts but has resisted full adoption, a process known as metrication.

In SI and older metric systems, prefixes indicate decimal multiples or fractions of a unit. For example, *kilo* means a factor of 1000 (10³), so a kilometre is 1000 metres; *milli* means a factor of 1/1000 (10⁻³), so a milligram is one thousandth of a gram.

The metric system is based on the metre, introduced in France in the 1790s. Its historical development led to the creation of SI in the mid-20th century under an international standards body. A key principle is selecting a set of independent base quantities—like length, mass, and time—from which all other quantities can be derived. Base units are now defined using fundamental natural phenomena rather than physical artefacts. Derived units, such as the square metre for area, are coherent, meaning they involve only products of powers of base units without extra factors. For any quantity, an extended set of larger and smaller units is defined by powers of ten.

The metric system is designed to be easy to learn and use, with units based on natural phenomena, decimal ratios, and a logical structure of base and derived units. It is also extensible: the governing body reviews and updates it as needed. For instance, the katal—a derived unit for catalytic activity equal to one mole per second—was added in 1999.

Each SI base unit must be realisable. Its definition includes a *mise en pratique* (practical realisation) describing at least one way to measure it. Where possible, definitions were developed so that any properly equipped laboratory could realise a standard without relying on an artefact held by another country.