SIsüsteemi

The SI system, officially the International System of Units (Système international d'unités, SI), is the world’s standard framework for measurement. It provides a coherent set of base and derived units used across science, industry, and commerce, enabling consistent communication and comparison of quantities such as length, mass, time, electric current, temperature, amount of substance, and luminous intensity.

The system is built on seven base units: metre for length, kilogram for mass, second for time, ampere for electric current, kelvin for thermodynamic temperature, mole for amount of substance, and candela for luminous intensity. From these base units, a wide range of derived units is formed, such as the newton (force), joule (energy), watt (power), pascal (pressure), and volt (electric potential). SI prefixes (nano-, micro-, milli-, kilo-, etc.) indicate decimal factors and allow expressing very large or very small quantities conveniently.

Since 2019, the SI base units have been defined by fixed values of fundamental constants rather than physical artefacts. The metre is defined by the speed of light in vacuum, the second by the cesium-133 hyperfine transition, the kilogram by the Planck constant, the ampere by the elementary charge, the kelvin by the Boltzmann constant, the mole by the Avogadro constant, and the candela by a luminous efficacy definition. The SI is maintained by the BIPM under the General Conference on Weights and Measures and the CIPM, with national metrology institutes adopting its definitions.

The SI is the prevailing standard for scientific research and most sectors of the global economy. While widely adopted, some regions retain traditional or supplementary units in everyday life.