Proposed revision of SI unit classifications: how the candela, mole, kelvin differ conceptually from the base units of kilogram, meter, second, ampere | CU Experts

Abstract; The 2019 revision of the International System of Units (SI) set exact values for four defining natural constants, allowing all base and derived units to be defined exactly. While the distinction between base and derived units remains valuable, particularly in education where it helps students grasp the SI’s structure more easily, we argue for clarifying the classification of these base units. Currently, the SI defines seven base units, but these fall into two fundamentally different subsets. The first subset—the kilogram (kg), meter (m), second (s), and ampere (A)—forms a foundational set of physics units from which all other units can be derived. The second subset—the candela (cd), kelvin (K), and mole (mol)—serves a different role. We propose recognizing this difference by classifying the candela, kelvin, and mole as a distinct category, possibly called ‘scale-spanning units’. While such a revision would not affect the accuracy of measurement, it would help students and newcomers to metrology to attain better clarity of understanding of the International System of Units.

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