Architecture for software-assisted quantity calculus

A quantity value, such as 5 kg, consists of a number and a reference (often an International System of Units (SI) unit) that together express the magnitude of a quantity. Many software libraries, packages, and ontologies that implement “quantities and units” functions are available. Although all of them begin with SI and associated practices, they differ in how they address issues such as ad hoc counting units, ratios of two quantities of the same kind, and uncertainty. This short article describes an architecture that addresses the complete set of functions in a simple and consistent fashion. Its goal is to encourage more convergent thinking about the functions and the underlying concepts so that the many disparate implementations, present and future, will become more consistent with one another.