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 and the concise summary is also available in Maori (PDF 245KB).

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.