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SI units

The redefinition of the SI units

The impact of the SI redefinition

In November 2018, at the General Conference on Weights and Measures, from 20 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.

Find out more about the SI units

Why do we need to change the SI?

As science advances, ever more accurate measurements are both required and achievable. But this improving accuracy needs to be enabled by measurement standards and their definitions.

Continuously improving the definitions of the units ultimately makes it possible to have tighter tolerances and less waste. For example, gears will fit together better and therefore function more efficiently and manufacturing will be able to rely on the dimensions of parts to fit together.   

The kilogram is the last SI base unit to be defined in terms of a human-made artefact, the International Prototype of the Kilogram (IPK). By definition the IPK always weighs one kilogram exactly. However, studies of closely similar copies tell us that the mass of the IPK is almost certainly changing … minutely. This implies a tiny but known change in the values of all masses. For mass, and for all units, we need to rule out this type of problem.

Constants of nature, such as the speed of light, are unchanging over time and space and we can give them exact values. Because of this, these constants provide the most stable and exact way to define all SI base units into the future.

What does the redefinition mean in practice?

Making this revision to the SI is a profound change in approach, that will underlie all measurements in science and more widely. But in everyday life it will appear that not much has changed. The new units will be the same size as previously but defined more precisely. The changes will ensure that the SI definitions remain robust for the future, ready for advancements in science and technology.

The new definitions impact four of the base units:

  • The kilogram – will be defined in terms of the Planck constant (h)
  • The ampere – will be defined in terms of the elementary charge (e)
  • The kelvin – will be defined in terms of the Boltzmann constant (k)
  • The mole – will be defined in terms of the Avogadro constant (NA)

Ensuring a seamless transition

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. 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