National Physical Laboratory

Measurement Units

SI Base Units

The SI (Système International d'Unités) is a globally agreed system of units, with seven base units.

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:

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:

The definitions of the SI units have a continuing history of change:


Fundamental Constants and Units

The fundamental physical constants, such as the speed of light, the Planck constant and the mass of the electron provide a system of natural units.

However, these must be related to the SI units by experiment. This experimental work is a global effort mostly undertaken in national standards laboratories to which NPL contributes. The constants provide the link between the SI units and theory and also between one part of physics and the SI and another.

For more information, a review article describing the background to the change to units based on fundamental constants is available.

NPL has activity in the Planck constant (watt balance) Rydberg constant (Hydrogen spectroscopy) Stefan-Boltzman constant (ARD).


Recommended Values of the Constants

A list of values and uncertainties of the most frequently used constants to CODATA Recommended Values (2005) is available.

These values are taken from the recommended values of the constants which are produced by the CODATA Task Group on Fundamental Constants, based on a review of all the available data. The latest review is available at the CODATA fundamental constants page at NIST. This should be consulted for values of the less frequently used constants or for covariances between the constants.

Kaye & Laby Online is a valuable resource

SI, Units & Constants FAQs