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International System of Units (SI)

By Robert Sheldon

What is the International System of Units (SI)?

The International System of Units is a global standard for expressing the magnitudes or quantities of important natural phenomena. Also referred to as the metric system, the System of Units is commonly abbreviated as SI, which comes from the original French name, Système international d'unités. The SI standard builds on an earlier system of measurement called the meter-kilogram-second (MKS) system.

The Bureau international des poids et mesures (BIPM) is responsible for promoting and describing the SI standard. Known as the International Bureau of Weights and Measures in English, the organization was established in 1875 and operates under the supervision of the International Committee for Weights and Measures (CIPM). The CIPM comes under the authority of the Conférence générale des poids et mesures (CGPM), also known as the General Conference on Weights and Measures.

At the heart of the SI standard is a set of seven defining constraints that serve as a foundation for all units of measurement specified in the SI standard.

According to SI documentation, the constants provide a "fundamental, stable and universal reference that simultaneously allows for practical realizations with the smallest uncertainties." All units identified in the SI standard can be derived from these seven constants.

International System of Units (SI) base units

Prior to 2018, seven base units provided the foundation for the SI standard. In addition, the standard defined multiple derived units. However, the standard now builds on its seven constants, and all defined units are derived from those constants. That said, the standard retained the concept of seven base units, along with the additional units derived from the base units.

All SI units can be expressed in terms of standard multiple or fractional quantities, as well as directly. Multiple and fractional SI units are defined by prefix multipliers and powers of 10 ranging from 10-24 to 1024. The seven base units are defined as follows:

The SI standard also includes units derived from the base units. The derived units are defined as products of powers of the base units. For example, one derived unit is the newton, which can be expressed in terms of base units as 1 kg m/s2. Other derived units include the hertz, the pascal (unit of pressure or stress), the ohm, the farad, the joule, the coulomb, the tesla, the lumen, the becquerel, the siemens, the volt and the watt.

These units are included in our Table of Physical Units.

14 Sep 2022

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