The International System of Units (SI) is the modern metric system built on seven base units — metre, kilogram, second, ampere, kelvin, mole and candela — from which all other units are derived; since 2019 every base unit is defined by fixing an exact value of a fundamental constant of nature.The International System of Units (SI) is the globally agreed framework for measurement, built on seven base units: the metre for length, kilogram for mass, second for time, ampere for electric current, kelvin for temperature, mole for amount of substance, and candela for luminous intensity. Every other unit, such as the newton for force or the joule for energy, is derived from these seven. The system ensures that a kilogram in Paris is the same as a kilogram in Tokyo, enabling science, trade, and technology to speak a common quantitative language.
The system evolved from the metric system introduced during the French Revolution in the 1790s. The modern SI was established in 1960 by the General Conference on Weights and Measures, and the shift to constant-based definitions was completed in 2019.
Since 2019, each base unit is defined by fixing an exact numerical value of a fundamental constant of nature. For example, the second is defined by the frequency of the cesium-133 atom's microwave transition, the metre by the speed of light, and the kilogram by the Planck constant. This means the units are tied to unchanging properties of the universe rather than to physical artifacts, making them universally reproducible in any well-equipped laboratory.
SI units underpin all scientific research, engineering, medicine, and international commerce, from calibrating medical devices to defining the watt in power grids.
The kilogram was the last base unit to be defined by a physical object — a platinum-iridium cylinder kept in a vault near Paris — until 2019, when it was redefined using the Planck constant.
