Logo of the Physikalisch-Technische Bundesanstalt

The revision of the International System of Units


Aer years of research at the major metrology institutes, the international community has agreed on the revision of the International System of Units (SI) – this revision came into force on 20 May 2019, which was World Metrology Day.

Tis day marked a turning point in the development of the physical and technical units of measurement, as the units have now been put on the most solid ground imaginable from a physical point of view: Their definitions are now based on a set of natural constants with defined values. In principle, the idea of defining units of measurement on the basis of natural constants is not new. What began 50 years ago with the definition of the second by means of atomic clocks, and went on over 30 years ago with the definition of the meter with the aid of the speed of light, will now continue for all of the units in the SI. Four other constants have now taken on the leading roles: the Planck constant h, the Avogadro constant NA, the Boltzmann constant k and the charge of an electron e.


In preparation for the revision of the SI, extensive experiments had taken place in metrology laboratories in previous years to measure these very constants as well as possible. The redefinitions were voted for unanimously at the General Conference of the Metre Convention held in Versailles back in November 2018.