National Physical Laboratory

The Measure of All Things

Formally agreed by the 11th General Conference on Weights and Measures (CGPM) in 1960, the SI is at the centre of all modern science and technology. The definition and realisation of the base and derived units is an active research topic for metrologists with more precise methods being introduced as they become available.

There are two classes of units in the SI: base units and derived units. The base units provide the reference used to define all the measurement units of the system, whilst the derived units are products of base units and are used as measures of derived quantities:

Ampere Candela Kelvin Kilogram Metre Mole Second

SI Bots

Redefining the kilogram

Stored in a basement at NPL is the UK's National Standard Kilogram, a copy of the international prototype and the standard for all mass measurements in the UK. But researchers at NPL and elsewhere are working hard to make the international prototype obsolete.

There are recommendations as to how to use SI units. SI prefixes are used to form decimal multiples and submultiples of the units:

Some non-SI units are still widely used:



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