The International System of Units, the SI, has been used around the world as the preferred system of units, the basic language for science, technology, industry and trade since it was established in 1960 by a resolution at the 11th meeting of the Conférence Générale des Poids et Mesures, the CGPM (known in English as the General Conference on Weights and Measures).
This brochure is published by the Bureau International des Poids et Mesures, the BIPM (known in English as the International Bureau of Weights and Measures) to promote and explain the SI. It lists the most significant Resolutions of the CGPM and decisions of the Comité International des Poids et Mesures, the CIPM (known in English as the International Committee on Weights and Measures) that concern the metric system going back to the 1st meeting of the CGPM in 1889.
The SI has always been a practical and dynamic system that has evolved to exploit the latest scientific and technological developments. In particular, the tremendous advances in atomic physics and quantum metrology made over the last 50 years have enabled the definitions of the second, the meter, and the practical representation of the electrical units to take advantage of atomic and quantum phenomena to achieve levels of accuracy for realizing the respective units limited only by our technical capability and not by the definitions themselves. These advances in science together with developments in measurement technology have enabled changes to the SI which have been promoted and explained in the previous editions of this brochure.
This 9th edition of the SI brochure has been prepared following the adoption by the 26th meeting of the CGPM of a set of far-reaching changes. The meeting introduced a new approach to articulating the definitions of the units in general, and of the seven base units in particular, by fixing the numerical values of seven “defining” constants. Among them are fundamental constants of nature such as the Planck constant and the speed of light, so that the definitions are based on and represent our present understanding of the laws of physics. For the first time, a complete set of definitions is available that does not make reference to any artifact standards, material properties or measurement descriptions. These changes enable the realization of all units with an accuracy that is ultimately limited only by the quantum structure of nature and our technical abilities but not by the definitions themselves. Any valid equation of physics relating the defining constants to a unit can be used to realize the unit, thus creating opportunities for innovation, realization everywhere with increasing accuracy as technology proceeds. Thus, this redefinition marks a significant and historic step forward.
The changes were agreed by the CGPM in November 2018 with effect from May 20th 2019, a date chosen because it is World Metrology Day, the day when the Meter Convention was signed in 1875. While the future impact of the changes will be far reaching, great attention has been paid to ensure that these definitions are consistent with those in place at the time the change was implemented.
We draw attention to the fact that since its establishment in 1960, the International System of Units has always been referred to as “the SI” in its shortened form. This principle has been maintained in the eight previous editions of this brochure and was reaffirmed in Resolution 1 adopted at the 26th meeting of the CGPM, which also confirmed that the title of this brochure is simply “The International System of Units”. This consistency of reference to the SI reflects the efforts of the CGPM and the CIPM to ensure the continuity of the values of measurements expressed in SI units through each change that has been made.
The text of this brochure has been prepared in order to provide a full description of the SI and to provide some historical background. It also has four appendices:
We conclude by expressing our thanks to the members of the Comité Consultatif des Unités of the CIPM, the CCU (known in English as the Consultative Committee for Units), who were responsible for drafting this brochure. Both the CCU and the CIPM have approved the final text.
Note on the text:
The 22nd meeting of the CGPM decided, in 2003, following a decision of the CIPM in 1997, that “the symbol for the decimal marker shall be either the point on the line or the comma on the line.” Following this decision, and following custom in the two languages, in this edition the point on the line is used as a decimal marker in the English text, and a comma on the line is used in the French text. This has no implication for the translation of the decimal marker into other languages. Small spelling variations occur in the language of the English speaking countries (for instance, “metre” and “meter,” “litre” and “liter”). In this respect, the English text presented here follows the ISO/IEC 80000 series Quantities and units. However, the symbols for SI units used in this brochure are the same in all languages.
Readers should note that the official record of the meetings of the CGPM and the sessions of the CIPM is that of the French text. This brochure provides the text in English, but when an authoritative reference is required or when there is doubt about the interpretation of the text the French should be used.
 Editors’ note: The 9th CGPM in 1948 initiated the study that led to the formal establishment of the SI by the 11th CGPM in 1960.
 Editors’ note: The spelling of English words is in accordance with the United States Government Printing Office Style Manual, which follows Webster's Third New International Dictionary rather than the Oxford Dictionary.