Revised definitions of the SI base units, adopted at the 26th General Conference on Weights and Measures (CGPM), that came into force on 20 May 2019
The 2019 SI redefinition grounded all seven base units in fixed natural constants, eliminating dependence on physical artefacts for the first time.
Key Facts
- Decision date
- 16 November 2018 (26th CGPM)
- Effective date
- 20 May 2019
- Units redefined in 2019
- Kilogram, ampere, kelvin, mole
- Defining constants added
- Planck, elementary charge, Boltzmann, Avogadro
- Previous major SI revision
- 1960
- Total SI defining constants after revision
- 7
By the Numbers
Location
Cause → Event → Consequence
The prototype kilogram and its copies showed small relative mass variations over time, and several SI units such as the kelvin were defined by measurements difficult to reproduce precisely. Decades of research eventually measured fundamental physical constants to sufficient accuracy relative to the old definitions, satisfying the agreed conditions for reform.
On 16 November 2018, the 26th General Conference on Weights and Measures unanimously approved redefining the kilogram, ampere, kelvin, and mole by fixing exact numerical values for the Planck constant, elementary charge, Boltzmann constant, and Avogadro constant respectively. The changes took effect on 20 May 2019, the 144th anniversary of the Metre Convention.
The International System of Units became wholly derivable from natural phenomena, with no definition dependent on a physical artefact. All seven base units are now grounded in seven defining constants, improving long-term stability and precision for scientific measurement worldwide, though some authors raised concerns about the severed link between the dalton, kilogram, mole, and Avogadro constant.