SI base unit
The SI base units are the seven fundamental units of measurement in the International System of Units (SI), which provide the basis for defining all other units in this globally standardized coherent system adopted in 1960. These units correspond to the basic physical quantities of time (second, s), length (metre, m), mass (kilogram, kg), electric current (ampere, A), thermodynamic temperature (kelvin, K), amount of substance (mole, mol), and luminous intensity (candela, cd), selected for their historical importance and dimensional independence.[1][2] Since the 2019 revision of the SI, effective from 20 May 2019, all base units are defined by assigning exact numerical values to seven key physical constants—the caesium frequency ΔνCs, speed of light c, Planck constant h, elementary charge e, Boltzmann constant k, Avogadro constant NA, and luminous efficacy Kcd—ensuring the system's definitions are invariant and linked directly to universal phenomena rather than artifacts or specific experimental realizations.[1][3] This revision, approved by the 26th General Conference on Weights and Measures in 2018, updated the definitions of the kilogram, ampere, kelvin, and mole to align with this constant-based framework, while the second, metre, and candela had already been so defined.[4][2] The definitions of the SI base units are as follows: This structure allows derived SI units, such as the newton for force or the joule for energy, to be expressed as products of powers of these base units without additional constants, promoting consistency in scientific, industrial, and everyday measurements worldwide.[1]Overview
Definition
SI base units are the foundational components of the International System of Units (SI), serving as the primary standards for measuring seven mutually independent base quantities; all other SI units, known as derived units, are formed by combining these base units through multiplication and division without introducing any numerical factors other than unity.[1] This structure ensures a coherent system where derived quantities, such as velocity or energy, can be expressed precisely in terms of the base quantities without ambiguity or loss of dimensional consistency.[12] The key criterion for designating a unit as an SI base unit is its association with a base quantity that cannot be derived from combinations of other quantities using the laws of physics, thereby maintaining dimensional independence and avoiding the introduction of arbitrary constants that could compromise measurement precision.[13] For instance, length and mass represent such base quantities, as they form irreducible dimensions essential for describing physical phenomena; in contrast, derived quantities like area or force are constructed from products or powers of these base quantities. At the 11th General Conference on Weights and Measures (CGPM) in 1960, the seven base quantities were selected to encompass the core aspects of physical reality relevant to science, technology, and everyday applications, providing a minimal yet comprehensive set for deriving all necessary measurements.[14] This choice reflects a balance between historical conventions and the need for practical universality, ensuring that the SI system supports advancements across diverse fields without requiring additional independent units.[15]Role in measurement
SI base units serve as the foundational elements of the International System of Units (SI), enabling the coherent derivation of all other SI units through simple multiplication and division without the need for conversion factors other than unity. This coherence arises because derived units are expressed as products of powers of the base units, ensuring that equations involving physical quantities maintain dimensional consistency and numerical simplicity. For instance, units such as area or velocity are formed directly from combinations like length squared or length per time, respectively, promoting a unified framework for measurements across various scales and applications.[2][16] The adoption of SI base units facilitates unambiguous communication of measurements in international trade, science, and technology, where precision and standardization are paramount to avoid errors and ensure interoperability. By providing a common language for quantifying physical quantities, these units support global commerce—such as in manufacturing specifications and regulatory compliance—and advance scientific collaboration by allowing researchers worldwide to replicate experiments and compare results reliably. This standardization is essential for industries ranging from electronics to pharmaceuticals, where inconsistent units could lead to costly misunderstandings or safety risks.[17][18] In dimensional analysis, SI base units form the basis for expressing any physical quantity as a product of powers of these fundamental units, enabling the verification of equation validity and the prediction of relationships between variables. This approach underscores the independence of the base quantities—such as length, mass, and time—allowing complex derived quantities to be broken down into their dimensional components for analytical purposes. By rooting all measurements in this set of base units, dimensional analysis becomes a powerful tool for ensuring theoretical consistency in fields like engineering and physics. The use of a single, carefully selected set of SI base units minimizes redundancy in the measurement system, eliminating the need for multiple arbitrary scales and fostering universality across disciplines including physics, chemistry, and engineering. This streamlined structure enhances efficiency in calculations and data interpretation, as professionals can rely on a shared set of references without discipline-specific adjustments. Ultimately, it promotes a cohesive global metrology ecosystem that supports innovation and equitable access to standardized knowledge.[2][19]The Seven Base Units
Physical quantities
The International System of Units (SI) is founded on seven base physical quantities that are regarded as dimensionally independent, serving as the foundation for defining all other physical quantities through derived units. These quantities were selected to encompass the essential aspects of measurement in physics, chemistry, and related sciences without redundancy or interdependence. The seven base quantities are: length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity.[1] Length is a fundamental quantity describing the extent of space between points, essential for geometry, mechanics, and positioning. Mass quantifies the amount of matter in a body, providing the basis for concepts like inertia and gravitational interaction. Time measures the duration of events and intervals between occurrences, underpinning dynamics and periodicity. Electric current represents the flow of electric charge, forming the core for electromagnetic phenomena and electrical engineering. Thermodynamic temperature indicates the degree of hotness or coldness of a system, critical for heat transfer and thermal equilibrium. Amount of substance counts the number of specified elementary entities, such as atoms or molecules, vital for stoichiometry and chemical reactions. Luminous intensity assesses the brightness of a light source in a given direction, accounting for human visual sensitivity in photometry.[18] These quantities are chosen because they are mutually independent—none can be derived from the others—and collectively allow the coherent expression of virtually all measurable physical properties across disciplines, from mechanical motion to quantum interactions, through products and quotients. This minimal set ensures universality and simplicity in scientific communication and experimentation.[1][12] The selection of these seven base quantities has remained unchanged since their complete establishment at the 14th General Conference on Weights and Measures (CGPM) in 1971, providing long-term stability for global metrology despite ongoing refinements in unit definitions.[20]Symbols and units
The seven SI base units, each corresponding to a fundamental physical quantity as outlined in the previous section on physical quantities, are standardized with specific names and symbols to ensure consistency in scientific communication. These are presented in the following table:| Base quantity | Unit name | Unit symbol |
|---|---|---|
| length | metre | m |
| mass | kilogram | kg |
| time | second | s |
| electric current | ampere | A |
| thermodynamic temperature | kelvin | K |
| amount of substance | mole | mol |
| luminous intensity | candela | cd |