International System of Units (SI)
The International System of Units (SI) is the modern form of the metric system and the world’s most widely used measurement system. It is built from seven base units, from which all other units are derived.
The seven SI base units
Every physical quantity in SI can be expressed using seven base units. Since the 2019 redefinition, each is defined by fixing the numerical value of a fundamental physical constant rather than a physical artefact.
| Quantity | Unit | Symbol | Definition |
|---|---|---|---|
| Time | second | s | Defined by fixing the caesium-133 hyperfine transition frequency Δν_Cs at exactly 9 192 631 770 Hz. |
| Length | metre | m | Defined by fixing the speed of light c at exactly 299 792 458 m/s, together with the second. |
| Mass | kilogram | kg | Defined by fixing the Planck constant h at exactly 6.626 070 15×10⁻³⁴ J·s, with the metre and second. |
| Electric current | ampere | A | Defined by fixing the elementary charge e at exactly 1.602 176 634×10⁻¹⁹ C, together with the second. |
| Temperature | kelvin | K | Defined by fixing the Boltzmann constant k at exactly 1.380 649×10⁻²³ J/K. |
| Amount of substance | mole | mol | Defined as exactly 6.022 140 76×10²³ elementary entities, the fixed value of the Avogadro constant N_A. |
| Luminous intensity | candela | cd | Defined by fixing the luminous efficacy of 540 THz radiation, K_cd, at exactly 683 lm/W. |
The 2019 redefinition by physical constants
On 20 May 2019, the SI base units were redefined so that they no longer depend on any physical object (such as the platinum-iridium kilogram prototype). Instead, seven defining constants — the caesium frequency Δν_Cs, the speed of light c, the Planck constant h, the elementary charge e, the Boltzmann constant k, the Avogadro constant N_A, and the luminous efficacy K_cd — are fixed exactly. This makes the units stable, universal and reproducible in any laboratory.
Derived units
Derived units are formed by combining base units. Many have special names; for example, the newton (N) for force is kg·m·s⁻², and the joule (J) for energy is kg·m²·s⁻².
| Unit | Symbol | In base units |
|---|---|---|
| hertz | Hz | s⁻¹ |
| newton | N | kg·m·s⁻² |
| pascal | Pa | kg·m⁻¹·s⁻² |
| joule | J | kg·m²·s⁻² |
| watt | W | kg·m²·s⁻³ |
| volt | V | kg·m²·s⁻³·A⁻¹ |
Frequently asked questions
What does SI stand for?
SI stands for the French “Système international d’unités”, the International System of Units — the modern, globally agreed form of the metric system.
Why was the kilogram redefined in 2019?
Until 2019 the kilogram was defined by a physical platinum-iridium cylinder whose mass drifted slightly over time. It is now defined by fixing the Planck constant h, so the unit is perfectly stable and can be realised anywhere.
How many SI base units are there?
There are seven SI base units: the second, metre, kilogram, ampere, kelvin, mole and candela. All other SI units are derived from these.