The authors describe the proposed figure of merit and use it to evaluate the relative merit of some specific single layer and stacked patch antennas. The proposed quantity gives the designer an understanding and limit of both the radiation efficiency and achievable impedance bandwidth of the printed antenna. Importantly, the quantity can be determined without the need for a numerically intensive full-wave analysis and can be applied, not only to single layer patches, but stacked geometries as well. The figure of merit is valid for probe-fed, edge-fed and proximity coupled patches. It also holds for aperture coupled patches as long as the slot is not too large. The theory behind this figure of merit is derived for a circular patch geometry, although it can easily be applied to other conductor shapes, such as rectangular or triangular. Results for circular microstrip patches using various materials are provided, including broadband stacked patches and stacked patches utilising high dielectric constant and low dielectric constant material combinations. From the results given it is evident that the derived figure of merit provides a good basis for the design of microstrip patch antennas.