SI units

The redefinition of the SI units

The impact of SI redefinition

In a landmark decision, made at the 26th meeting of the General Conference on Weights and Measures (CGPM), the BIPM's Member States voted to revise the International System of Units (SI). This means that the base units are now defined in terms of defining constants, which are the most stable references available for us to use.

Four of the base units acquired new definitions: the kilogram, ampere, kelvin and mole. The definitions of the other three base units were reworded to emphasise their dependence on their corresponding defining constants. The most substantial change is how the kilogram is defined. The kilogram is now 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

Changes in the International System of Units of measurement

Fiona Auty, Head of Government Relations and Corporate Communications at NPL

Read blog

Kilo redefinition presents strategic innovation opportunity for UK

Professor Ric Parker, Distinguished Visitor at the National Physical Laboratory, and former CTO of Rolls-Royce

Read blog

Supporting science learning in schools

To celebrate and explain the SI definitions, NPL has published 12 new schools posters describing the SI and measurement

School posters

Why did we need to change the SI?

More accurate measurement facilitates advances in science and technology, which in turn allows more accurate meaurement. 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 was the last SI base unit to be defined in terms of an 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 was almost certainly changing… minutely. This implied a tiny but known change in the values of all masses. For mass, and for all units, we needed 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?

The revision to the SI was 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 are 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 impacted four of the base units:

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