SI - International System of Units

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

Units for electrotechnical technology

Comparing what is comparable

IEC Technical Committee 25  prepares international standards on quantities and units to be used in electrical and electrotechnical technology. It also reviews the use of quantities and units in IEC Standards. These standards should, whenever possible, be based on the SI. They may be related to definitions, names, letter symbols and their use or to the relations in which these quantities and units appear, and to the signs and symbols used with them.

Close co-operation is required with IEC TC 1: Terminology, and technical liaison are maintained with the following organizations:

  • BIPM - International Bureau of Weights and Measures
  • ISO/TC 12 - Quantities, units, symbols, conversion factors
  • ITU - International Telecommunication Union
  • OIML - International Organization of Legal Metrology


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

The revised SI

The changes to the SI were unanimously approved during the 26th General Conference on Weights and Measures (CGPM) in November 2018 and became operational in May 2019.

The definitions of all base units are now explicitly based on constant quantities. The kilogram, the ampere, the kelvin, and the mole, in particular, are defined by setting exact numerical values for the Planck constant (h), the elementary electric charge (e), the Boltzmann constant (k), and the Avogadro constant (NA) respectively.

The new definitions aim to improve the universality and the stability of the SI, while ensuring continuity of existing instrument calibrations and measurement results. The numerical values of the defining constants have been carefully chosen so as to guarantee that the new definitions identify the same quantities as before.

In particular, the “1948 ampere” – which was when the previous definition of the ampere was agreed – and the “2019 ampere”, although defined in a completely different way, is substantially the same intensity of the electric current.