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SI units

The SI base units

The globally agreed system of measurements was formerly named the International System of Units (SI) in 1960. The SI covers units for every type of measurement, but at the heart of the SI is a set of seven units known as the ‘base units’.

kilogram (kg) Unit of measurement of mass
metre (m) Unit of measurement of length
second (s) Unit of measurement of time
ampere (A) Unit of measurement of electric current
kelvin (K) Unit of measurement of thermodynamic temperature
mole (mol) Unit of measurement of amount of substance
candela (cd) Unit of measurement of luminous intensity

This International System of Units is necessary to ensure that our everyday concepts of measurement, whether a metre or a second, remain comparable and consistent worldwide. Standardising such measurements not only helps to keep them consistent and accurate, but also helps society have confidence in information. For instance, the kilogram is used every day, and defining this quantity means that consumers can trust that the shop is really providing the amount they say they are. Having reliable information on climate change, pollution and medical diagnostics is important to society and builds trust and allows effective decisions to be made.

See what the redefinition in May 2019 will mean for the SI units

How are the units of measurement defined?

Historically, units of measurement were defined by physical objects or properties of materials. For example, the metre was defined by the length between lines engraved on a metal bar and the kilogram is still defined as the mass of a single cylinder of platinum-iridium metal – the International Prototype of the Kilogram (IPK).

In these two examples, the definition was also the physical form – the realisation – of the unit. However, these physical representations can change over time or in different environments. So, over the years, the definitions have been improved to be more stable and reproducible, and to meet the needs of today’s research and technological applications.

During the last century, scientists measured constants of nature, such as the speed of light and the Planck constant, with increasing accuracy. They discovered that these were more stable than physical objects. It became clear that these constants of nature could offer a new and more stable foundation for the SI.

See what impact the SI redefinition will have on calibration services

SI derived units
SI prefixes
SI conventions
Non-SI units