In this paper we derive a method to determine the worst-case radiated immunity of a given printed circuit board (PCB) layout, whose benefits are threefold. First, the worst-case performance could be used as a reference to compare the radiated immunity of two competing, schematically-identical-yet-electromagnetically-distinct layouts. Second, it could be used as a criterion to determine whether a given layout is able to observe the particular requirements of a given electromagnetic compatibility (EMC) standard. And finally, since this method is element-independent, it requires no modelling for complex circuit elements such as non-linear elements, logic gates, processors, etc. to interact with electromagnetic interference (EMI). Our method determines how much power could be delivered to the non-linear circuit elements of the PCB by an EMI under a worst-case scenario. This could be done, either for the entire PCB, or for specific parts and elements of interest, such as sensitive elements, logic gates, microprocessors, etc. While a fully analytical approach with a closed-form solution is developed for the former, the latter could be obtained through an analytical derivation followed by numerical maximization. These methods would enable the rapid and straightforward evaluation of the designs’ immunity at any stage, resulting in saving cost and time.