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SI units

The SI base units

The globally-agreed system of measurement units was formally 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 mass
metre (m) Unit of length
second (s) Unit of time
ampere (A) Unit of electric current
kelvin (K) Unit of thermodynamic temperature
mole (mol) Unit of amount of substance
candela (cd) Unit of luminous intensity

This International System of Units is necessary to ensure that our everyday measurements 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, mass is measured every day, and having agreement on the definition of the kilogram means that consumers can trust that the shop is really providing the mass they say they are. Equally, having reliable information on climate change, pollution and medical diagnostics is important to society and builds trust and allows effective decisions to be made.

Find out more about the redefinition of 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 was 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 realisation – the physical form – of the unit. However, these physical representations can change over time and are susceptible to damage or loss. So, over the years, the definitions have evolved to depend on constants of nature that are 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.

SI derived units
SI prefixes
SI conventions
Non-SI units

Defining constants

From 20 May, the SI units will be defined in terms of constants of nature, in which:

  • the unperturbed ground state hyperfine transition frequency of the caesium-133 atom Δν is 9 192 631 770 hertz, also known as the 'Cs frequency'
  • the speed of light in vacuum c is exactly 299 792 458 metres per second
  • the Planck constant h is exactly 6.626 070 15 × 10–34 joule seconds
  • the elementary charge e is exactly 1.602 176 634 ×10–19 coulombs
  • the Boltzmann constant k is exactly 1.380 649 × 10–23 joules per kelvin
  • the Avogadro constant NA is exactly 6.022 140 76 × 1023 reciprocal moles
  • the luminous efficacy of monochromatic radiation of frequency 540 ×1012 hertz Kcd, is exactly 683 lumens per watt