The seven SI base units: names, symbols and what they measure
A revision guide to the seven SI base units used in science and measurement.
The seven SI base units are the second, metre, kilogram, ampere, kelvin, mole and candela. They correspond respectively to time, length, mass, electric current, thermodynamic temperature, amount of substance and luminous intensity.
Memorise the pairs
Time: second (s). Length: metre (m). Mass: kilogram (kg). Electric current: ampere (A). Thermodynamic temperature: kelvin (K). Amount of substance: mole (mol). Luminous intensity: candela (cd).
The SI is a coherent system of units. Many other units are derived from these base units, such as the newton for force and the joule for energy. Derived units combine base units mathematically.
Exam trap
The kilogram is the SI base unit for mass, while the newton measures force. Weight is a force caused by gravity, so mass and weight are not interchangeable in physics.
Use the official SI definitions for precision; memory tricks can help revision but should not replace the unit names and symbols.
Editorial note
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Frequently asked questions
Which SI base unit measures amount of substance?
The mole (mol).