The International System of Units (SI)

The International System of Units (SI)

The globally agreed system of measurement used across science, industry, trade and society.

 

A common language for measurement

What is the SI?

The International System of Units, or the SI, is the internationally agreed system for measurement. It provides a consistent and universal framework for expressing measurement results, enabling measurement results to be compared reliably across countries, disciplines and applications.

 

Why the SI matters

Measurements behind everyday decisions

Many of the decisions we make every day depend on measurements we rarely notice. From medical diagnoses and the food we buy to transport, manufacturing, climate monitoring and scientific research, reliable measurements underpin activities that affect people, economies and societies.

The SI provides a common reference for these measurements, making it possible for measurement results to be understood and compared across countries and fields of application.

 

The seven SI base units

The SI includes seven base units, each associated with a fundamental quantity. The symbols below — such as s for the second, m for the metre and kg for the kilogram — are the internationally agreed symbols used to express these units consistently in measurement results.

s

second
time

m

metre
length

kg

kilogram
mass

A

ampere
electric current

K

kelvin
temperature

mol

mole
amount of substance

cd

candela
luminous intensity

How the SI works

From scientific definitions to measurements in practice

The SI establishes internationally agreed definitions for measurement units. To make these units available for practical measurement, the definitions are realized through experimental methods, with internationally agreed guidance supporting their implementation.

Together, these steps connect the scientific definitions of the SI to measurements made in laboratories and other applications around the world.

Definitions

Definitions are fixed and universal. They establish the basis of the SI by fixing the values of defining constants.

Realizations

Realizations are experimental methods used to make SI units available in practice and establish traceability to the SI.

Mises en pratique

The Consultative Committees of the CIPM provide internationally agreed guidance on practical realizations of SI units.

 

The scientific foundation of the SI

The seven defining constants

The seven base units describe the principal base quantities of the SI. The defining constants play a different role: they provide the scientific foundation for the definitions of the SI units.

Since 20 May 2019, all SI units have been defined by fixing the numerical values of seven defining constants. These provide a stable and universal reference for the SI, while allowing measurement methods to evolve with advances in science and technology.

This approach allows improved measurement techniques to be adopted without changing the definitions of the units themselves.

The fixed values that define the SI

The modern SI is defined by assigning exact numerical values to seven defining constants. Together, these fixed values provide the foundation from which the SI units can be realized.

ΔνCs

the unperturbed ground state hyperfine transition frequency of the caesium-133 atom

c

the speed of light in vacuum

h

the Planck constant

e

the elementary charge

k

the Boltzmann constant

NA

the Avogadro constant

Kcd

the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz

Read more about the defining constants of the SI →

 

International coordination

Making the SI work worldwide

The SI is established and maintained through decisions of the General Conference on Weights and Measures (CGPM), supported by the International Committee for Weights and Measures (CIPM) and coordinated by the BIPM.

The BIPM works with the CIPM Consultative Committees and National Metrology Institutes to support the implementation of the SI, coordinate international comparisons and promote globally comparable measurement results.

Measurements that can be compared worldwide

Through this international system, laboratories can establish traceability to the SI and demonstrate the comparability of measurement results across countries and economies.