SI units

Impact on calibration services

The impact on calibration services of the SI redefinition

In November 2018, at the General Conference on Weights and Measures, from May 2019, all the base units will be defined in terms of defining constants, which are the most stable things available for us to use.

The new definitions impact four of the base units: the kilogram, ampere, kelvin and mole. The most substantial change will be to how mass is disseminated within national measurement institutes (NMIs). The kilogram will defined in terms of the Planck constant rather than via the International Prototype of the Kilogram held at the International Bureau of Weights and Measures in France.

The redefinition of the SI will be the system of units in which:

  • the unperturbed ground state hyperfine transition frequency of the caesium-133 atom Δν 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 of monochromatic radiation of frequency 540 ×1012 hertz Kcd, is exactly 683 lumens per watt

The changes will impact a small number of high level electrical calibration services, but most customers using our calibration services can expect a negligible impact. NPL will be sending out more information to all our customers after the redefinition has been confirmed in November 2018. We expect a seamless transition for companies using our measurement and calibration services, and the new definitions will form the foundation for improved measurement for decades to come.

Information for users about the proposed revision of the SI

Information for users of NPL calibration services for electrical quantities

Read more about the SI redefinition