An open-access paper published in Metrologia reviews the changes to the SI that will come into force on 20 May 2019. In particular, the definition of the kilogram will change for the first time since 1889. The definitions of the ampere, the kelvin and the mole will be changd at the same time.

Following three decades of intensive work by the world-wide metrology community, the 26th General Conference on Weights and Measures (CGPM) agreed this major revision of the SI at its meeting in November 2018. The paper includes as an Annex the complete text of Resolution 1 of the 26th CGPM (2018) – On the revision of the International System of Units (the SI).

This Resolution presents the definitions of the base units in a new format that highlights the link between each unit and a defined value of an associated constant. The physical concepts underlying the definitions of the kilogram, the ampere, the kelvin and the mole have been changed. The new definition of the kilogram is of particular importance because it eliminates the last definition referring to an artefact (the international prototype of the kilogram, IPK). In this way, the new definitions use the rules of nature to create the rules of measurement and tie measurements at the atomic and quantum scales to those at the macroscopic level.

The podcast of the historic decision to revise the SI can be viewed on the BIPM's YouTube channel.

• Stock M., Davis R., de Mirandés E., Milton M.J.T., The revision of the SI – the result of three decades of progress in metrology, Metrologia, 2019, 56, 022001