The elegance of the SI

1. The great political foundation of the International System of Units is that it has been developed by international agreement by the General Conference on Weights and Measures meeting four-six yearly since 1875.
2. The great intellectual foundation of the SI is that it is based on a formal quantity calculus (ISO 31:1992), using seven mutually independent base quantities.
   
3. The great physical foundation of the SI is that its base units are mostly rooted in the physical world, defined using various universal physical constants.
4. The great mathematical foundation of the SI is that it is a coherent system of units, so that physical equations like E=mc² don’t require conversion factors. In other words, if you use the base unit for “c” (m/s) and the base unit for “m” (kg), the result by definition is in joules, the SI unit of energy. This is far easier, and therefore less prone to error, than using the foot-pound-second (FPS) system, where a factor is required depending on whether you want the answer in ergs, BTU’s or “megatons of TNT”.
5. The great arithmetical foundation of the SI is that it is decimal-based, so you don’t have to remember how many pounds in a UK or US ton, how many fluid ounces in a UK or US gallon, and all the other complicated diversity of FPS units.
6. The great rigour of the SI is that it has a defined symbol for every unit, as distinct from the plethora of abbreviations invented for many FPS units (e.g. mi/hr, mph; psi, lbf/in² etc).
7. The great scalability of the SI derives from having a defined prefix for each multiple and submultiple of a unit (e.g. n=10^-9 , G= 10⁹), allowing a huge range of values to be expressed concisely.