Definition · Plain-language
SI base units
The SI base units are the seven fundamental units of the International System of Units, from which every other unit of measurement is derived.
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The seven base units
The International System of Units rests on seven base units, each tied to a physical quantity: the second for time, the metre for length, the kilogram for mass, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance, and the candela for luminous intensity. These seven are deliberately kept to a minimum, because every other SI unit — the newton, the joule, the volt, the watt — is a derived unit defined as a combination of base units. Holding the foundation small and coherent is what lets the whole system fit together without contradiction.
Defined by constants of nature, not objects
Until 2019, the kilogram was defined by a single platinum-iridium cylinder kept near Paris — a physical artefact that could, in principle, change mass over time. The 2019 redefinition swept the last such artefact away. Now every base unit is fixed by assigning an exact numerical value to a fundamental constant: the kilogram to the Planck constant, the metre to the speed of light, the second to a caesium atom’s frequency, and the ampere to the elementary charge. Because constants of nature are the same everywhere and always, the units are now stable and reproducible in any suitably equipped laboratory.
Base units versus derived units
A derived unit is any unit expressed as a product or ratio of base units. Speed is metres per second; force, the newton, is kilogram metres per second squared; energy, the joule, is a newton metre. Many derived units carry special names for convenience — the pascal, hertz, volt and ohm among them — but each can be written out in base units. This structure means there is one coherent answer to "what is the SI unit of …" for any physical quantity, which is precisely the property that makes scientific results comparable worldwide.
Key facts
At a glance
- Definition: the seven fundamental units of the International System of Units
- The seven: second, metre, kilogram, ampere, kelvin, mole, candela
- Quantities: time, length, mass, current, temperature, amount, luminous intensity
- Since 2019: each is defined by a fixed constant of nature, not a physical object
- Derived units: all other SI units (newton, joule, volt) are built from these
- Authority: defined and maintained by the BIPM in the SI Brochure
Common misconceptions
What people often get wrong
Often heard: The kilogram is still defined by a metal cylinder kept in Paris.
Actually: Not since 2019. The kilogram is now defined by fixing the numerical value of the Planck constant, so it no longer depends on any physical object that could change over time.
Often heard: The litre and the degree Celsius are SI base units.
Actually: They are not. The seven base units are the second, metre, kilogram, ampere, kelvin, mole and candela. The litre and degree Celsius are accepted or derived units built on top of these.
Often heard: There are dozens of fundamental units in the SI.
Actually: There are exactly seven base units. Every other SI unit — and there are many — is a derived unit, defined as a combination of those seven.
Going deeper
