International System of Units (SI): overview, base units, and uses

Core characteristics and base units

SI is designed to be coherent: derived units can be expressed algebraically from base units without additional numerical factors. It uses a decimal prefix system to scale units by powers of ten, making conversion straightforward. The SI defines seven base quantities and their units:

• metre — unit of length;
• kilogram — unit of mass;
• second — unit of time;
• ampere — unit of electric current;
• kelvin — unit of thermodynamic temperature;
• mole — unit for amount of substance;
• candela — unit of luminous intensity.

These base units are mutually independent and, when combined, produce the SI derived units used to express quantities such as volume, energy, pressure, and velocity. Examples of common derived unit names include the newton (force), joule (energy), pascal (pressure), and hertz (frequency).

Development and modern definition

The system was established by international agreement in the 20th century to unify metric practice and simplify scientific communication. Over time SI definitions have evolved from artefacts and material standards toward definitions based on fundamental physical constants. In recent revisions the metre has been tied to the speed of light, while the kilogram, ampere, kelvin and mole have been redefined in terms of exact values of constants such as the Planck constant, the elementary charge, the Boltzmann constant and Avogadro's number. These changes make SI stable and reproducible in different laboratories without reliance on a single physical object.

Derived units, prefixes and usage

Derived units are formed algebraically from base units (for example, m/s for velocity or kg·m/s² for force). SI prefixes provide standardized decimal multiples and submultiples — familiar examples are kilo- (10³), milli- (10⁻³) and micro- (10⁻⁶). This consistency supports clear communication across disciplines, from engineering and manufacturing to chemistry and medicine.

Global adoption and notable points

SI is used almost everywhere in the world. While a few countries do not use SI as their sole legal measurement system, Myanmar, Liberia and the United States still retain non-SI customary units in some official or widespread civilian contexts; however, SI is widely used in science, health care, international trade and many areas of government even in those countries. The system's coherence, reliance on constants, and international oversight promote interoperability of measurements and facilitate technological development.

Institutions and standards

International organizations maintain and update SI definitions and guidance. National metrology institutes implement realizations of the units and provide calibration services. For accessible introductions and technical details consult authoritative sources on the metric tradition and metrology, or follow standard-setting bodies for current recommendations (metric system, SI, Système International d'unités, system of measurement, metre, kilogram, second, ampere, kelvin, mole, candela, volume, energy, pressure, velocity, Myanmar, Liberia, United States).

Because SI definitions now rest on invariant constants, the system supports highly precise measurement and international comparability — essential features for science, commerce and technology in a globalized world.