Main title

SI zone

International System of Units

SI zone

The International System of Units, or SI (from its French name, “Système international d'unités”), is a system of measurement units, based on the International System of Quantities (ISQ), defined in the ISO and IEC 80000 series of standards. The SI comprises base and derived units corresponding to base and derived quantities of the ISQ respectively.


Base units

Seven base units form the core of the SI:

  • the second (s) for duration (i.e., extension of time);
  • the metre (m) for length (i.e., extension of space);
  • the kilogram (kg) for mass;
  • 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 from the base units via the equations that relate the corresponding quantities in the ISQ. For example, the unit of velocity in the SI is metre per second (m/s), because the derived quantity velocity in the ISQ is defined as length divided by duration. A number of derived units in the SI have been given special names and symbols, such as:

  • the unit for force: newton (N = kg · m/s2);
  • units for electric quantities: 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 magnetic quantities: weber (W, for magnetic flux), tesla (T, for magnetic flux density), and henry (H, for inductance);
  • units for light-related quantities: 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 related to duration: minute (min), hour (h), and day (d);
  • units used in everyday life: litre (l, for volume) and tonne (metric ton, t, for mass).

In the SI, decimal prefixes for multiples and submultiples are used, ranging from yotta (Y, 1024) to yocto (y, 10–24).
Furthermore, the SI acknowledges the binary multiples that IEC standardized in IEC 60027-2, Letter symbols to be used in electrical technology — Part 2: Telecommunications and electronics.

For example, the prefix kibi (Ki) is 210 = 1 024 instead of kilo (k) which is 103 = 1 000.