The Seven SI Base Units Explained
Source: BIPM. Published: 2025-02-15.
What Are Base Units and Why Do They Matter?
The International System of Units, universally abbreviated SI from the French Système International d'unités, is the modern form of the metric system and the world's most widely adopted measurement framework. At its foundation are seven base units — the metre, kilogram, second, ampere, kelvin, mole, and candela. These are not base units because they are somehow more fundamental than other physical quantities; they are base units by convention, chosen to span a set of dimensions from which all other units of measurement can be derived. Velocity, for example, is derived from the metre and the second. Force is derived from the kilogram, metre, and second. The choice of which seven quantities to treat as base is partly historical and partly practical, but the result is a coherent system in which any measurement in any scientific or engineering discipline can be expressed in terms of these seven quantities and the mathematical relationships between them.
The Seven: A Quick Tour
The metre (m) measures length. The kilogram (kg) measures mass. The second (s) measures time. The ampere (A) measures electric current. The kelvin (K) measures thermodynamic temperature. The mole (mol) measures amount of substance — how many elementary entities (atoms, molecules, ions) are present. The candela (cd) measures luminous intensity — the power of light in a given direction, weighted by the human eye's sensitivity. Of these seven, the metre, kilogram, and second are the most commonly encountered in everyday life. The ampere, kelvin, mole, and candela are more specialised but equally indispensable in their respective domains — electrical engineering, thermodynamics, chemistry, and photometry. Together they underpin every quantitative statement in science, engineering, medicine, and commerce worldwide.
Physical Artefacts and Their Problems
For much of the SI's history, several base units were defined by reference to physical objects or reproducible experimental procedures with measurable uncertainties. The kilogram was defined by a platinum-iridium cylinder; the metre was originally defined by a similar bar. These physical definitions caused practical problems. A physical object can be damaged, lost, or altered over time. Measurements made against different national copies of the kilogram prototype revealed small discrepancies that grew over decades, without any way to determine which copy had drifted. Artefact-based definitions also limited the precision of the unit: the kilogram could only be known as precisely as the best comparison between prototypes, a measurement constrained by practical and instrumental limits. The scientific community spent decades developing alternatives, ultimately converging on a solution that grounded every base unit in fundamental physical constants.
The 2019 Revision: Constants Over Objects
On May 20, 2019, a comprehensive revision of the SI came into effect, simultaneously redefining four of the seven base units. The kilogram was redefined through Planck's constant. The ampere was redefined through the elementary charge of an electron. The kelvin was redefined through the Boltzmann constant. The mole was redefined through Avogadro's constant. The metre, second, and candela had already been redefined in terms of constants in earlier revisions. The result was a complete system in which all seven base units are now defined by fixing the numerical values of seven fundamental constants of nature: the speed of light, Planck's constant, the elementary charge, the Boltzmann constant, Avogadro's constant, the luminous efficacy of 540 THz radiation, and the hyperfine transition frequency of cesium-133. These constants are the same everywhere in the universe, making the SI definitions truly universal.
Derived Units Built on the Seven
The power of the base unit framework lies in the derived units it generates. The pascal (Pa), the SI unit of pressure, equals one newton per square metre — itself a combination of kg⋅m⋅s⁻². The watt (W), the unit of power, equals one joule per second. The joule (J), the unit of energy, equals one kg⋅m²⋅s⁻². The volt (V), the unit of electric potential, combines kilograms, metres, seconds, and amperes. In total, the SI formally names twenty-two derived units with special names, from the hertz (frequency) to the sievert (dose equivalent of ionising radiation). Beyond these named units, an unlimited number of quantities — velocity, density, concentration, magnetic flux density — can be expressed as products and quotients of base units with appropriate exponents. This mathematical structure makes the SI self-consistent: there are no conversion factors between related quantities within the system.
The SI System and Global Trade
The SI is not merely an academic convenience — it is the legal basis for trade, safety regulation, and scientific communication in most of the world. The Metre Convention of 1875 committed signatory nations to a common system of measurement, and today the BIPM has 64 member states representing the vast majority of global economic activity. Uniform measurement underpins pharmaceutical dosing, structural engineering tolerances, food labelling, financial transactions measured in milliseconds, and the interoperability of GPS satellites maintained by different national agencies. Wherever units appear — on a product label, in a medical prescription, in an aircraft manual — they ultimately trace back to the SI base units and, through them, to fixed values of physical constants. The seven base units are, in a very real sense, the common language of quantitative human activity.
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