Redressing grievances with the treatment of dimensionless quantities in SI

The International System of Units (SI) and its associated methods adhere to a design principle in which all kinds of quantities are described in terms of a standard set of dimensions. It is a corollary of that principle that quantities having no extent in any of those dimensions (dimensionless quantities) will not be distinguishable as different kinds through dimensional analysis alone, and the formality of quantity calculus will be of no help in preventing mistakes where different kinds of dimensionless quantities are confused. This corollary has led to controversies involving abuse of the unit hertz and elimination of radians and steradians as being equivalent to the number 1. In addition, official rejection of the use of so-called counting units in conjunction with accepted units has been repeatedly criticized and is often ignored. The proposed resolution is to extend quantity calculus to discern different kinds of dimensionless quantities as well as it currently discerns different kinds of dimensional quantities. The extended system provides a formal interpretation of commonly-used workarounds in terms that are compatible with SI. Should the extended quantity calculus become accepted by consensus in the future, a guide to its use would expand the set of references provided by the Joint Committee for Guides in Metrology UCGM), which currently lacks a definitive reference on quantity calculus. Published by Elsevier Ltd.