Progress towards redefining the International System of Units in 2018

Last weeks (20 October) meeting of the International Committee for Weights and Measures (CIPM) resulted in a resolution recommending the redefinition of four base units of the International System of Units (SI): the kilogram, the ampere, the kelvin and the mole.

These recommendations will then go before the General Conference on Weights and Measures (CGPM), which oversees the SI, for final worldwide agreement in November 2018. If the submission passes the vote, the four units will be based on exact values of four fundamental constants. The changes would take effect on World Metrology Day, 20 May 2019.

"The decision by the CIPM to propose a resolution to the 26th CGPM on the redefinition of the base units in the SI brings the vision of an International System of Units based on invariant constants of nature one step closer to realisation. In taking this decision, the CIPM acknowledges the outstanding contributions by many metrologists and the commitments of their institutes to undertake the long-term experiments necessary to make this decision possible," explains Dr Barry Inglis, President of the CIPM.

What is the revision to the SI?
In the revised SI, the kilogram, the ampere, the kelvin and the mole will be redefined in terms of constants. These new definitions will be based on fixed numerical values of:

  • The Planck constant (h),
  • Elementary charge (e),
  • The Boltzmann constant (k),
  • The Avogadro constant (NA), respectively.

The remaining three base units, the metre, the second and the candela, are already defined in terms of constants. The definitions of all the SI base units will then be uniformly expressed using an explicit-constant formulation, and specific mises en pratiques (practical ways) are being drawn up for experimentally realising the base unit redefinitions.

The result will be a simpler and more fundamental definition of the entire SI and dispenses with the last of the unit definitions based on a material artefact - the International Prototype Kilogram.

An important step forward for measurement

Using constants of nature to define international measurement units allows industry and the scientific community to accurately scale their measurements from the smallest to the largest quantities therefore meeting modern measurement requirements. It will tie key measurements at the atomic and quantum scales to those at the macroscopic level with great precision.

Just as the redefinition of the second helped enable GPS navigation, the redefined SI is expected to foster the development of new technologies we have yet to imagine, whilst maintaining continuity for practical users.

From the redefinition, the SI will be the system of units in which:

  • The unperturbed ground state hyperfine transition frequency of the caesium-133 atom ΔνCs is 9 192 631 770 hertz
  • The speed of light in vacuum c is exactly 299 792 458 metres per second
  • The Planck constant h is exactly 6.626 070 15 × 10-34 joule seconds
  • The elementary charge e is exactly 1.602 176 634 × 10-19 coulombs
  • The Boltzmann constant k is exactly 1.380 649 × 10-23 joules per kelvin
  • The Avogadro constant NA is exactly 6.022 140 76 × 1023 reciprocal moles
  • The luminous efficacy Kcd of monochromatic radiation of frequency 540 × 1012 Hz is exactly 683 lumens per watt

For more details, visit and

Select your area of interest