The International System of Units

The International System of Units (SI, from the French Système International d'Unités) is the modern form of the metric system and the globally adopted framework for quantitative measurement in science, engineering, commerce, and public administration. It defines seven base units from which all other measurement units in the system are derived, and it specifies a coherent set of prefixes for expressing multiples and submultiples of those units. Adopted formally by the General Conference on Weights and Measures (CGPM) in 1960, the SI has been refined through successive revisions, with the most recent and fundamental redefinition taking effect on 20 May 2019.

The SI is administered by the International Bureau of Weights and Measures (BIPM), headquartered in Sèvres, France, which coordinates the maintenance of national measurement standards and the comparisons between them. Interoperability of measurements across borders depends on SI-traceable calibration chains, making the system foundational to international trade, regulatory compliance, and scientific publication.

The Seven Base Units

The SI defines seven base units, each corresponding to a distinct physical quantity. They are the metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (thermodynamic temperature), mole (amount of substance), and candela (luminous intensity). Until 2019, several of these units were defined by physical artifacts or empirical measurements subject to drift. The 2019 redefinition tied all seven base units to fixed numerical values of fundamental physical constants: the Planck constant h, the elementary charge e, the Boltzmann constant k, and the Avogadro constant N_A, among others. The NIST International System of Units reference documents the current definitions, uncertainty budgets for realizing each unit, and the approved symbols and spelling conventions used in scientific publication worldwide.

Derived Units, Prefixes, and Special Names

From the seven base units, the SI constructs a coherent set of derived units to express all other measurable quantities. Twenty-two derived units carry special names and symbols approved by the CGPM: among these are the hertz (frequency), newton (force), pascal (pressure), volt (electric potential), ohm (resistance), and watt (power). The SI prefix system assigns standard multiplier prefixes from quetta (10^30) to quecto (10^-30), allowing engineers and scientists to express measurements across many orders of magnitude without switching unit systems. The 2022 CGPM added the prefixes ronna, ronto, quetta, and quecto to address data volumes in computing and observations in cosmology. The coherence of the derived unit system means that equations in mechanics, electromagnetism, and thermodynamics hold without conversion factors when quantities are expressed in SI units, a property that simplifies both calculation and dimensional analysis.

Governance and the 2019 Redefinition

The SI is maintained through a governance structure in which the BIPM provides secretariat services and measurement science, the CGPM serves as the supreme authority meeting every four years, and the International Committee for Weights and Measures (CIPM) oversees ongoing technical work through a network of consultative committees. National metrology institutes, including NIST (United States), PTB (Germany), NPL (United Kingdom), and NMIJ (Japan), serve as the primary realizers of SI units within their respective countries and contribute clock comparisons, laser frequency standards, and Kibble balance measurements that feed into the internationally recognized values. The PTB description of SI unit realisation illustrates how a national laboratory connects primary standards to the global framework through international comparison campaigns. The rationale and technical basis for the 2019 redefinition are described in the BIPM's documentation of the revised SI, which explains how defining exact values for physical constants eliminated the last remaining artifact-dependent unit (the kilogram) and made the SI fully independent of physical objects that can change over time.

Applications

The International System of Units has applications in a wide range of disciplines, including:

• Calibration and traceability chains for manufacturing quality control and metrology
• Scientific publication standards requiring SI-consistent reporting of measurements
• Legal metrology and trade compliance for weights and measures in commerce
• Electrical and electronics engineering design and circuit analysis
• Pharmaceutical dosing, clinical laboratory measurements, and medical device standards