Units, Measurements & Scientific Instruments22 Concepts & Facts

What Is the International System of Units (SI)? The 7 Base Units & 2019 Redefinition

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The International System of Units, officially abbreviated worldwide as SI from the French Le Système International d'Unités, functions as the universally adopted language of modern physical measurement, science, engineering, and commerce. Established in 1960 during the 11th General Conference on Weights and Measures (CGPM), the SI framework is the direct evolutionary descendant of the metric system created during the French Revolution. Operating under the enduring diplomatic mandate of the Metre Convention of 1875, the SI system is maintained and periodically revised by the International Bureau of Weights and Measures (BIPM) based in Sèvres, France.

At the core of the SI framework are seven fundamental base units, from which all other mechanical, thermal, electrical, and optical quantities are mathematically derived. These seven base units are the second (unit of time, symbol s), the metre (length, m), the kilogram (mass, kg), the ampere (electric current, A), the kelvin (thermodynamic temperature, K), the mole (amount of substance, mol), and the candela (luminous intensity, cd). Derived units are constructed through algebraic combinations of these base units and often carry special names honoring legendary scientists, such as the newton for force, the joule for energy, the watt for power, the pascal for pressure, and the volt for electric potential.

The SI system underwent an epochal scientific transformation on 20 May 2019 (World Metrology Day), following unanimous approval at the 26th CGPM in November 2018. For more than a century, the kilogram had been defined by a physical human artifact: "Le Grand K," a cylinder made of platinum-iridium alloy stored under triple vacuum glass bells in Sèvres. The 2019 redefinition eliminated all physical artifacts, defining every single SI base unit strictly in terms of immutable, fundamental physical constants of the universe. Today, the kilogram is fixed via the Planck constant, the ampere via the elementary charge, the kelvin via the Boltzmann constant, and the mole via the Avogadro constant. In India, primary measurement standards and Indian Standard Time (IST) are maintained by the CSIR-National Physical Laboratory (CSIR-NPL) in New Delhi.

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The International System of Units (SI) was officially established in 1960 by the 11th General Conference on Weights and Measures (CGPM).

SI is administered by the International Bureau of Weights and Measures (BIPM) located at Sèvres, France, under the 1875 Metre Convention.

The SI system is built upon exactly seven fundamental base units representing distinct physical dimensions.

The Second (s) measures time, defined by the caesium-133 ground-state hyperfine transition frequency (9,192,631,770 Hz).

The Metre (m) measures length, defined by fixing the speed of light in vacuum (c) at exactly 299,792,458 meters per second.

The Kilogram (kg) measures mass, redefined in 2019 using the Planck constant (h = 6.62607015 × 10⁻³⁴ J·s).

The Ampere (A) measures electric current, redefined in 2019 using the elementary charge (e = 1.602176634 × 10⁻¹⁹ C).

The Kelvin (K) measures thermodynamic temperature, redefined in 2019 using the Boltzmann constant (k = 1.380649 × 10⁻²³ J/K).

The Mole (mol) measures amount of substance, redefined in 2019 using the Avogadro constant (N_A = 6.02214076 × 10²³ mol⁻¹).

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The Candela (cd) measures luminous intensity, defined by the luminous efficacy of monochromatic radiation at 540 × 10¹² Hz.

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On 20 May 2019, all seven SI base units became completely decoupled from manufactured physical artifacts.

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Before 2019, the kilogram was the last unit defined by a physical object: 'Le Grand K', a platinum-iridium cylinder cast in 1879.

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A Kibble balance (watt balance) allows researchers to measure mass with extreme precision by balancing weight against electromagnetic forces.

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Derived SI units with special names include Newton (force), Joule (energy), Watt (power), Pascal (pressure), and Hertz (frequency).

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Electrical derived units include Coulomb (charge), Volt (potential), Ohm (resistance), Farad (capacitance), and Tesla (magnetic flux density).

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Radiation units include Becquerel (radioactive decay), Gray (absorbed radiation dose), and Sievert (equivalent biological dose).

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SI rules stipulate that unit symbols named after people are capitalized (N for Newton, J for Joule), while full names are written lowercase.

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Prefixes range across 30 orders of magnitude, expanded in 2022 to include ronna (10²⁷), quetta (10³⁰), ronto (10⁻²⁷), and quecto (10⁻³⁰).

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CSIR-National Physical Laboratory (CSIR-NPL) in New Delhi functions as India's National Metrology Institute under the Legal Metrology Act, 2009.

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CSIR-NPL operates a bank of atomic caesium clocks that generates Indian Standard Time (IST), accurate to within a few nanoseconds.

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Two supplementary units formerly used in SI—radian (plane angle) and steradian (solid angle)—were reclassified as dimensionless derived units in 1995.

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SI units provide the non-negotiable quantitative foundation for all modern science, international trade contracts, and engineering tolerances.

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Educator's Insight

The International System of Units, universally known as SI, is the globally standardized metric language of modern science and commerce. Established in 1960, the system is built upon seven fundamental base units: the second for time, meter for length, kilogram for mass, ampere for electric current, kelvin for temperature, mole for substance amount, and candela for luminous intensity. In 2019, all seven units were formally redefined using invariant fundamental physical constants.

For UPSC Prelims and SSC science papers, questions often test the 2019 SI redefinition. Remember that physical metal prototypes, like the International Prototype of the Kilogram stored in France, were completely retired in favor of universal constants like the Planck constant and Boltzmann constant. Watch out for multiple-choice traps regarding temperature: kelvin is the absolute SI base unit, not Celsius. Also note that supplementary units like radians and steradians are classified as dimensionless derived units.

What Is the International System of Units (SI)? The 7 Base Units & 2019 Redefinition

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