The International System of Units, or SI (from its French name Système international d'unités) is a system of metric weights and measures. It comprises, base and derived units.

Base units

Seven base units form the core of the SI:

• the metre (m) for length;
• the kilogram (kg) for mass;
• the second (s) for time;
• the ampere (A) for electric current;
• the kelvin (K) for thermodynamic temperature;
• the mole (mol) for amount of substance;
• the candela (cd) for luminous intensity.

Derived units

Derived units are obtained in terms of the base units according to a system of equations relating the corresponding quantities. The SI derived units are obtained from the SI base units according to the International System of Quantities, or ISQ. The ISQ has all the time implicitly been the basis of the SI, but the concept ISQ has not until recently been explicitly recognized. For example, the SI derived unit of speed is metre per second (m/s), because the derived quantity speed in the ISQ is defined as length divided by time.

22 SI derived units have been given special names and symbols, such as newton (N = kg · m/s²) as the unit for force, including:

• units for electricity: coulomb (C, for electric charge), volt (V, for electric potential and electric tension), farad (F, for capacitance), ohm (Ω for resistance) and siemens (S, for conductance);
• units for magnetism: weber (W, for magnetic flux), tesla (T, for magnetic flux density), and henry (H, for inductance);
• units for light: lumen (lm, for luminous flux) and lux (lx, for illuminance).

Additional units

In addition to the SI units, some additional units have also been adopted for international use, including:

• units of time: minute (min), hour (h), and day (d);
• metric units used in everyday life: litre (l, for for volume) and tonne (metric ton, t, for mass).

Prefixes

In the SI decimal prefixes for multiples and submultiples, ranging from yotta (Y) 10²⁴ to yocto (y) 10^–24 are used.

IEC has also standardized prefixes for binary multiples (where kibi (Ki) means 2¹⁰ = 1 024 instead of kilo (k) which means 10³ = 1 000) in its International Standard IEC 60027-2, Letter symbols to be used in electrical technology — Part 2: Telecommunications and electronics.