International System of Units
A standard set of units of measurement for use in scientific literature worldwide. The newest and most widespread version of the metric system.
SI: the global metric standard for all technical measurement
The International System of Units, known as SI from its French name Système International d'Unités, is the unified framework of base units and derived units used across science, engineering, and industry worldwide. It replaced the older CGS (centimetre-gram-second) and MKS (metre-kilogram-second) systems and has become the legal standard for measurement in nearly every country except the United States, Liberia, and Myanmar. In industrial practice, SI compliance is often mandatory for export documentation, regulatory approval, and cross-border trade.
The SI defines seven base units from which all others derive: the metre (m) for length, kilogram (kg) for mass, second (s) for time, ampere (A) for electric current, kelvin (K) for thermodynamic temperature, mole (mol) for amount of substance, and candela (cd) for luminous intensity. From these bases, derived units such as the newton (N) for force, pascal (Pa) for pressure, joule (J) for energy, and watt (W) for power are constructed using multiplication and division. This coherent system eliminates arbitrary conversion factors; one newton always equals exactly one kilogram-metre per second squared.
Prefixes and practical scale
SI uses standardised prefixes to express multiples and submultiples: kilo (10³), mega (10⁶), giga (10⁹), milli (10⁻³), micro (10⁻⁶), nano (10⁻⁹), and others. This decimal scaling makes it straightforward to work with very large or very small quantities without introducing separate unit names. A machinist measuring a tolerance of 0.05 mm writes it as 50 micrometres, maintaining the same unit family rather than switching notation.
In inspection and quality control, SI units are the reference standard for all traceable measurement. Calibration certificates, dimensional drawings, and test reports must state results in SI units or show conversion factors explicitly. Deviations from SI practice introduce rounding errors and ambiguity; a pressure reading given in atmospheres or bar rather than pascals complicates verification against design specifications and international standards such as ISO or DIN.
The SI system is maintained by the International Bureau of Weights and Measures (BIPM) in Sèvres, France, and is refined periodically through the General Conference on Weights and Measures. Recent redefinitions have tied all base units to fundamental physical constants rather than material artifacts, improving reproducibility. Any industrial metrology laboratory claiming traceability to national standards must ultimately trace to SI.