SI Units

The SI underwent a major revision in 2019 that was implemented by the international metrology community. This global change involved moving away from material artefacts (e.g., the kilogram prototype in Paris), and instead defining all base units in terms of a set of constants of nature.

Four of the base units – the kilogram, ampere, kelvin, and mole – have been redefined based on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (k), and the Avogadro constant (N_A), respectively. The three remaining base units – the second, metre and candela – were already defined by physical constants, so only minor edits were made to their definitions.

This unifying approach has resulted in a simpler and more fundamental definition of the entire SI, and it builds new measurement foundations for the scientific and industrial communities of the 21st century. It directly links measurements at the atomic (and quantum) scales to those at the macroscopic level in areas that had previously relied on indirect methods.

The new definitions improve the SI without significantly changing the size of any units, thus ensuring continuity with existing measurements, and these changes to the SI will provide a springboard for future innovation.

Further information on the SI and the changes can be found on the BIPM website. You can download a handy factsheet on the SI redefinition here.

Today, metrology is a field that allows global economies and nation states to share a common language of trade, research and development, but it was born out of rebellion. It was the creation of the decimal metric system on 22 June 1799, in the throes of the French Revolution, that kickstarted the development of the present International System of Units. Based on the metre and the kilogram, and represented by two platinum standards safeguarded in the Archives de la République in Paris, it was the first step on the path of globally-standardised measurements.

The introduction of the British CGS system in 1874, based on three units – the centimetre, gram and second – marked further development. But arguably, the next landmark in global metrology history came with the Metre Convention, which was signed by representatives of 17 nations on 20 May 1875. Two new prototypes of the metre and kilogram were internationally sanctioned, and when combined with the astronomical second as the unit of time, they established a mechanical unit system called the MKS. A fourth dimension – the ampere, used as a measure of electrical current – was finally added to this system in 1949. The kelvin and the candela were added as base units for thermodynamic temperature and luminous intensity in 1960.

The name International System of Units, with the abbreviation SI, was given to the extended system in 1960. And in 1971, the current version of the SI was completed by adding the mole as the base unit for amount of substance.