In classical Newtonian mechanics, equations and formulas never change form. The same thing cannot be said about the equations and formulas of electromagnetic theory, which often change form when converted from one system of units to another. This work aims to provide the technical professional with an explanation of how to convert electromagnetic equations from one system of units to another. The level is geared to professionals and students who have a sound understanding of electricity and magnetism. It explains all five systems of electromagnetic units in depth, four connected to the centimetre-gram-second (cgs) system of mechanical units and one connected to the metre-kilogram-second (mks) system of mechanical units - commonly called the ""Systeme International"".