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The International System of Units (SI)

The International System of Units, SI (Système International d'Unités), is a foundation for modern society and a prerequisite for science, technological development, and international trade.

As the National Metrology Institute of Sweden, RISE is responsible for the central quantities within the SI system.

The SI units

The SI is made up of seven base units, many derived units, and a collection of prefixes.

Kilogram

The unit of mass.

Symbol: kg

The National Laboratory for Mass

Mole

The unit of amount of substance.

Symbol: mol

The National Laboratory for Chemical Metrology

Candela

The unit of luminous intensity.

Symbol: cd

The National Laboratory for Photometry and Radiometry

Other units

All other SI units can be derived from the seven base units by simple multiplication and division. Some derived units have special names, such as hertz for frequency, newton for force, pascal for pressure, and joule for energy. Other units do not have their own names but are named based on the constituent units, such as metres per second (m/s) for speed and kilograms per cubic meter (kg/m3) for density.

More National Laboratories:

Defined by constants of nature

What is a kilogram or a metre? Historically, units of measurement were defined based on artifacts, physical objects. The metre was defined as the distance between two lines on a metal bar and the kilogram as the international kilogram prototype, IPK (a metal cylinder of platinum and iridium). All measurements were traced back to these physical objects at the International Bureau of Weights and Measures in Paris (BIPM). This meant that the units’ definition was also its realisation. In metrology realising means to bring the measurement unit from its definition into the real world. A kilogram was by definition the IPK, and a kilogram in reality was also the IPK.

One of the problems with using artefacts is that physical objects can be changed or affected, such as through mishandling, air pollution or theft. Over the years, therefore, all units have been redefined based on constants of nature. The kilogram was redefined in 2019. This means that the definitions of the measurement units are independent of the realisations. For each unit, there are so-called mises en pratique, approved methods, for how the units can be realised. Since the definitions are no longer linked to the realisations, new and better mises en pratique can be developed as technology advances.

The work of tying the units of measurement to defining constants of nature has been recognised with Nobel prizes in several areas, which highlights the level of research in the field but also the importance that is placed internationally on having a uniform and robust system of measurement units.

Read more about the constants of nature that define the SI at BIPM.

Jan Johansson

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Jan Johansson

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+46 10 516 55 04

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