How to Define the Units of the Revised SI Starting from Seven Constants with Fixed Numerical Values

Abstract

As part of a revision to the International System of Units (SI) approved in2018 and to take effect in May 2019, the seven base units will be defined bygiving fixed numerical values to seven defining constants. This article shows howthe definitions of all seven base units can be derived efficiently from thedefining constants, with the result appearing as a table. The table’s form makesevident a number of connections between the defining constants and the base units.Appendices show how the same methodology could have been used to define the samebase units in the present SI, as well as the mathematics that underpins themethodology. Since the base units are now defined in terms of constants, then allunits in the SI are now defined in terms of those constants.