SI (International System of Units)

SI, abbreviated from the French term 'Système international d'unités', is the most widely used system of measurement in science, commerce and law worldwide. It defines standardized units for measuring mass, length, time, electric current, temperature, quantity of substance and other physical quantities.

The system rests on seven base units: the metre (length), the kilogram (mass), the second (time), the ampere (electric current), the kelvin (temperature), the mole (quantity of substance) and the candela (luminous intensity). Other units—such as the newton, joule, watt and others—are derived from these base units.

Brief history

The foundation of SI originates from the metric system, introduced in France at the end of the 18th century. In subsequent centuries, international agreements were concluded establishing bodies to jointly maintain measurement standards. The name 'Système international d'unités' was adopted at the general conferences on weights and measures during the 20th century.

Definition based on physical constants

In the most recent revision, SI units have been defined in terms of permanent physical constants rather than tangible objects. For example, the kilogram was previously defined by a physical artefact kept in France; it is now defined by Planck's constant. Similarly, the metre is tied to the speed of light, and the second is tied to the oscillation frequency of the caesium atom.

Prefixes

SI uses decimal prefixes to express smaller or larger measurements: kilo (thousand), mega (million), giga (billion), and conversely milli (one-thousandth), micro, nano and others. This allows the same unit to express vastly different magnitudes without introducing new units.