International System of Units

The International System of Units, known globally by the abbreviation SI, is the modern form of the metric system and the most widely used measurement framework in science, technology, industry and everyday life. Adopted almost universally, it is the only system of measurement with formal status across nearly all nations. Its development and continual coordination are overseen by the International Bureau of Weights and Measures (BIPM), headquartered near Paris.

Foundations and Structure of the System

At the core of the system lie seven SI base units, each corresponding to a fundamental physical quantity. These are:

• second (s) – time
• metre (m) – length
• kilogram (kg) – mass
• ampere (A) – electric current
• kelvin (K) – thermodynamic temperature
• mole (mol) – amount of substance
• candela (cd) – luminous intensity

From these base units the system generates a vast number of coherent derived units, defined as algebraic combinations of powers of the base units without any numerical factor. Examples include the metre per second for velocity and the pascal for pressure. Twenty-two derived units carry specially assigned names and symbols, such as the newton, joule and watt. The system, however, permits an unlimited number of coherent derived units as needed for scientific and technical applications.
To accommodate quantities of different magnitudes, the SI employs a structured set of twenty-four metric prefixes that indicate decimal multiples or submultiples of units. These prefixes range from very large powers of ten (such as quetta for 10³⁰) to very small ones (such as quecto for 10⁻³⁰). Combined with base or derived units, they generate non-coherent but convenient alternatives such as kilometre or milligram.

The Defining Constants

Modern SI definitions are built entirely on a set of seven defining constants, each assigned an exact numerical value. These constants form the conceptual foundation of the system and include:

• the speed of light in vacuum
• the caesium-133 hyperfine transition frequency
• the Planck constant
• the elementary charge
• the Boltzmann constant
• the Avogadro constant
• the luminous efficacy of monochromatic radiation

This approach removes dependence on physical artefacts and allows improvements in measurement technology without the need to redefine units. The system thus separates conceptual definitions from practical realisations, allowing more precise implementations over time.

Base Units and Their Definitions

Each base unit is defined in relation to one or more of the defining constants. For example, the kilogram is realised by fixing the Planck constant to an exact value. These definitions maintain continuity with earlier versions of the metric system while enabling superior measurement precision. The choice of seven base quantities is ultimately a matter of convention rather than necessity, as any complete set of independent quantities may serve similar purposes.
The second is defined by the caesium standard, the metre by the speed of light, the ampere by the elementary charge, the kelvin by the Boltzmann constant, the mole by the Avogadro constant and the candela by a specified luminous efficacy. This interconnected network of definitions ensures internal consistency throughout the measurement system.

Coherent and Derived Units

In a coherent system the equations governing physical quantities retain identical numerical forms when expressed in the system’s units. The SI preserves this property by defining coherent derived units through direct relations to base quantities. Velocity, force, pressure and similar concepts therefore adopt natural combinations of the base units without extraneous constants.
Some derived units have historically assigned special names—such as the radian and steradian—which do not themselves correspond to any specific base unit. These are maintained within the system for continuity and ease of communication.
Derived units may also be expressed in alternate valid forms. For instance, electrical resistance, measured in ohms, can be expressed through the ratio of volts to amperes. Similarly, electrical conductance uses the reciprocal unit, the siemens. This flexibility enables users to choose the most appropriate representation for a particular scientific or engineering context.

Metric Prefixes and Decimal Structure

A defining characteristic of the system is its reliance on a decimal structure. Conversions between units representing the same physical quantity always involve powers of ten. Prefixes such as kilo-, centi- and milli- create consistent and systematic relationships among unit magnitudes. As these prefixes attach inseparably to unit symbols, they can be manipulated algebraically—for example, combining km³ or mg·m⁻² as needed.
Prefixes are widely used both in everyday life and in specialised fields. Driving distances are typically measured in kilometres; temperatures may be reported in millikelvin in physics laboratories; data quantities use prefixes such as kilo- or mega- for clarity.

Development and Historical Context

The SI emerged from a long process of rationalising earlier metric systems, particularly the centimetre-gram-second (CGS) systems that had developed inconsistently across scientific disciplines. Conflicts between electromagnetic and electrostatic units and the lack of unified conventions motivated reform in the twentieth century.
The General Conference on Weights and Measures (CGPM), created through the 1875 Metre Convention, spearheaded international efforts to standardise units, nomenclature and measurement practices. After proposals introduced in 1948, the modern SI was formally established in 1960, drawing from the metre-kilogram-second (MKS) framework and refining it using scientific advances.

Conceptual Approach and Realisation

The redefinition of SI units in the twenty-first century reflects a broader shift toward abstraction and universality. The conceptual definitions are anchored in constants rather than physical objects or specific experimental setups. Realisations in laboratories—such as atomic clocks or watt balances—are designed to embody these definitions within acceptable measurement uncertainties.
This shift allows the system to evolve as new methods emerge without altering the foundational definitions. It also addresses challenges posed by artefact-based units, including degradation or loss, ensuring that the SI remains robust, reproducible and universally applicable.

Contemporary Role and Global Adoption

Today the International System of Units forms the bedrock of global measurement practice. Its clarity, precision and coherence enable reliable communication of scientific results, safe operation of technologies and consistent international trade. By providing a common language for expressing quantities, it facilitates collaboration across borders, disciplines and industries.