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SI base units
Beginner
Seven base units: metre, kilogram, second, ampere, kelvin, mole, candela.
The International System (SI) fixes seven base units: metre (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (amount) and candela (luminous intensity). Since 2019 all seven are defined from fixed values of natural constants, such as the speed of light and Planck's constant, so any lab can reproduce them without a physical artefact.
The metre is the distance light travels in 1/299,792,458 of a second, so the speed of light is exactly 299,792,458 m/s by definition.
Grams are not the SI base unit of mass; the kilogram is. This matters when you substitute into formulas like F = ma.
In more detail
The SI system is built on seven base units from which all other units are derived. Defining the base units precisely, often using fundamental constants, makes measurements reproducible worldwide.
Real-world uses
- Scientific papers and international trade use SI units so results can be compared directly.
- The definition of the second based on atomic clocks keeps GPS and telecoms synchronised.
- Laboratories and standards agencies compare every national measurement back to the SI base units, so a result in one country can be checked in another.
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Frequently asked questions
Why was the kilogram redefined?
In 2019 the kilogram was tied to the Planck constant instead of a physical metal cylinder, so it no longer depends on one object that could drift in mass.
Is the litre an SI unit?
No. The litre is accepted for use with SI, but the SI unit of volume is the cubic metre.
Are the SI base units fixed forever?
Since 2019 they have been defined by fixed values of physical constants, such as the speed of light and the Planck constant, so they no longer depend on a physical artefact.
Related topics in Measurement & Math Tools
What is physics? · Physical quantity · Derived units · Prefixes (milli, kilo, mega) · Scientific notation