It's more like a 2-level situation for most things with a little more structure than you might realize. There are "physical kinds" or meta types like "energy"

It's more like a 2-level situation for most things with a little more structure than you might realize. There are "physical kinds" or meta types like "energy" and then different concrete units those can be which allow conversion. The physical kind is a point in an N-dimensional rational space of exponents of base units, such as Mass * Distance ^2 * Time^-2 or (1,2,-2) for energy (maybe with some carry over ^0s for unused slots of a >3D meta-type space). Additive (like add, subtract, compare, and convert) are only meaningful at the same point in that lattice. If you are interested in learning more, this is all a part of dimensional analysis: https://en.wikipedia.org/wiki/Dimensional_analysis https://en.wikipedia.org/wiki/Dimensional_analysis . Even the dimensionality of that rational exponent space (the "unit system", if you will) is more of convenience / convention than fundamental. https://en.wikipedia.org/wiki/Planck_units https://en.wikipedia.org/wiki/Planck_units clobber most things away leaving only 1 dimension (and a tendency for complex rational exponents). Meanwhile, the SI unit system is very "dimension promiscuous" (mass, length, time, electric current, temperature, amount of substance, luminous intensity) [1] to avoid non-integer exponents (but you cannot really forever since as soon as some formula has some square some solution of it has some root that probably gets you a 1/2 exponent). There are some situations with not purely "conversion factor" scale offsets like thermal units (e.g. Celsius to Fahrenheit) that people often describe in math-ese as "affine conversions". Something like GNU Units attends these things, but they are kind of "conversion only" orphan step-children more than "real" types which "compose better". In physics formulae one will often only care about delta-Temperature, not levels, for example. EDIT: The implicit creation of a potentially brand new type any time you multiply | divide two extant things is, incidentally, why you need either a dynamic or a very powerful static type system to express these things. [1] https://en.wikipedia.org/wiki/International_System_of_Units https://en.wikipedia.org/wiki/International_System_of_Units