Sign up for NPL updates
Sign up for NPL updates

Receive regular emails from NPL to get a glimpse of our activities and see how our experts are informing and influencing scientific debate

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 Member States of the Metre Convention voted to revise the International System of Units (SI). This means that the base units are now defined in terms of constants that describe the natural world,, which are the most stable references available for us to use.

From 20 May 2019 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 as the mass of the International Prototype of the Kilogram held at the BIPM 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 measurement. But this improving accuracy needs to be enabled by stable measurement standards underpinned by agreed definitions of measurement units.

Continuously improving the definitions of the units ultimately makes it possible to have tighter tolerances, better efficiency 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, prior to 20 May 2019, the IPK always weighed 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 unknown change in the values of all masses. For the kilogram, and for all units, we needed to rule out this type of problem, caused by relying on physical artefacts as standards.

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

Find out about our current research on SI units

What does the redefinition mean in practice?

The revision to the SI was a profound change in approach, that will underpin 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 into the future, ready to accommodate advancements in science and technology, which will be able to be translated directly into improvements in the accuracy of measurement.

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


The Last Artifact from MontanaPBS is 90 minute film that follows the story of the kilogram. It features scientists from NPL and across the world who worked on the metrology that enabled this profound change to happen.

Watch The Last Artifact