Collapse calderas are common in many volcanic regions, but the mechanics of their formation is not well understood. I propose that collapse calderas may form as a result of the growth of associated shallow magma chambers. Excess magmatic pressure in a shallow chamber gives rise to tensile stresses at the surface, but the location and magnitude of the maximum tensile stress depend on the shape of and depth (d) to the top of the chamber. This model indicates that penny-shaped chambers, i.e., sill-like chambers of circular horizontal cross sections, are most likely to initiate collapse calderas. For such a chamber with horizontal radius a, the higher the a/d (radius/depth) ratio, the greater the probability of forming a collapse caldera.