Version 4.01 of the 9th edition of the SI Brochure has now been published.

The SI Brochure is the authoritative document describing the International System of Units (SI), the world's only internationally agreed system of units. The new version introduces revisions to the section on non-SI units and updates the presentation of informative notes throughout the publication.

Clarifying the role of non-SI units

The revised section on non-SI units responds to feedback received from stakeholders and clarifies how non-SI units are presented within the SI Brochure.

For the first time since the SI was formalized, the SI Brochure now makes explicit that non-SI units do not hold any special status within the SI. The revision also introduces a more inclusive list of non-SI units together with their conversion coefficients to SI units.

To help explain their inclusion, the listed units are organized into four indicative categories:

• Long-standing units of time and angle;
• Historical names for decimal multiples and submultiples of SI units;
• Internationally recognized units that are not decimal multiples or submultiples of SI units;
• Units used in specialized technical disciplines.

Supporting the digitalization of the SI

The update also incorporates the informative notes that previously appeared in the margins of the SI Brochure into the main body of the text.

This change supports the continued digitalization of the SI Brochure and improves the presentation of information across digital formats.