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SI Units

The International System of Units, the SI, is the internationally agreed basis for expressing measurements at all levels of precision and in all areas of science, technology, and human endeavour.

The authoritative description of the SI and its development is given in the SI brochure (8th ed.) available from the BIPM (Internatoinal Bureau of Weights and Measures) web site in both full and summary form.

The value of a quantity is expressed as the product of a number and a unit. The SI uses a set of well defined and easily accessible units universally agreed for the multitude of measurements that support today’s complex society.  The units have been chosen so that they are readily available to all, are constant throughout time and space, and are easy to realise with high accuracy.  The SI is founded on 7 base units which are considered to be independent of each other.  By combining these using equations expressing physical laws, derived units are created.  For each kind of quantity there is only one SI unit.

For more on the base and derived units follow the links to each measurement area.

Brief History

The creation of the decimal metric system at the time of the French Revolution and the subsequent deposition of two platinum standards representing the metre and the kilogram, on 22 June 1799, in the Archives de la République in Paris can be seen as the first step in the development of the present International System of Units.

After the signing of the Metre Convention on 20 May 1875, work began on the new international prototypes of the metre and kilogram. In 1889 these international prototypes, together with the astronomical second as the unit of time constituted a three-dimensional mechanical unit system.  With the base units metre, kilogram and second it was known as the MKS system.

In following years the ampere, the kelvin and the candela were added as base units, respectively, for electric current, thermodynamic temperature and luminous intensity. The name International System of Units, with the abbreviation SI, was given to the extended system in 1960.  In 1971 the current version of the SI was completed by adding the mole as the base unit for amount of substance.

Revision of the SI in 2018

The International System of Units (SI) will undergo a major revision in 2018 that will be agreed and implemented by the international metrology community.

The changes to the SI involve no longer relying on artefacts to derive some units (such as the kilogram prototype in Paris) and instead use fundamental physical constants.

The use of constants in nature enables you to link from the smallest to the largest measurement quantities. It will tie measurements at the atomic (and quantum) scales to those at the macroscopic level. This introduces the appeal of a fundamental (“quantum”) basis for the changes.

There will still be the same seven base units (second, metre, kilogram, ampere, kelvin, mole, and candela). Of these, the kilogram, ampere, kelvin and mole will be redefined by choosing exact numerical values for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively. The second, metre and candela are already defined by physical constants and it is only necessary to edit their present definitions.

The new definitions will improve the SI without changing the size of any units, thus ensuring continuity with present measurements.

The changes to the SI will provide a springboard for future innovation.

Further information on the changes can be found on the BIPM website.