International System of Units

The International System of Units, abbreviated SI from its French name, Système international d'unités, is a coordinated framework for expressing physical quantities. A measurement combines a numerical value with a unit: using the same definitions lets results be compared across laboratories, countries and fields. The SI is a decimal system related to the metric system, but it is more than a collection of familiar conversions: it also specifies how units are defined, written and realized in practice. The Bureau International des Poids et Mesures (BIPM), together with the General Conference on Weights and Measures (CGPM), the International Committee for Weights and Measures (CIPM) and national metrology institutes, coordinates this international framework.

The SI has seven base units, each associated with a base quantity: the second (s) for time, metre (m) for length, kilogram (kg) for mass, ampere (A) for electric current, kelvin (K) for thermodynamic temperature, mole (mol) for amount of substance, and candela (cd) for luminous intensity. Other SI units are derived by combining powers of the base units. For example, velocity is expressed in metres per second (m/s), while the newton, the unit of force, is kg·m/s². Some derived units have special names and symbols, including the joule for energy and the watt for power. Decimal prefixes such as kilo- and milli- express multiples or fractions of units without changing the underlying unit definition.

Since the 2019 revision, the SI is defined by fixing exact numerical values for seven defining constants of nature and physics. These include the speed of light in vacuum, the Planck constant, the elementary charge, the Boltzmann constant and the Avogadro constant. The 2018 CGPM decision, effective 20 May 2019, changed the definitions of the kilogram, ampere, kelvin and mole; for instance, the kilogram is now defined through the Planck constant rather than by reference to a single physical prototype. This makes the definitions independent of a particular object, while experiments still provide practical methods for realizing the units and tracing measurements back to the SI. Definitions are fixed; the experimental methods used to realize them can improve as technology advances.

The SI grew out of efforts to make measurement standards internationally consistent. The Metre Convention, signed in 1875, established the BIPM as a center for coordinating measurement standards. In 1960, the 11th CGPM adopted the International System of Units, expanding on the metre–kilogram–second system. International agreements have since updated the definitions as science and measurement technology developed. The major revision approved in November 2018 and effective in 2019 replaced definitions tied to physical artifacts or material properties with definitions based on fixed constants. The CGPM makes decisions on the SI, the CIPM supports and advises on their implementation, and the BIPM coordinates international measurement work with national metrology institutes.

A shared system helps ensure that a measurement made in one place can be understood and compared elsewhere. This comparability supports scientific research, manufacturing, international commerce, healthcare, transport and monitoring of the environment. The SI does not eliminate the need for careful measurement: instruments must be calibrated, uncertainty assessed and results reported with appropriate precision. Instead, it supplies common definitions and conventions that allow those results to be communicated and checked across disciplines and borders.