The thermal resistance of requisite thin dielectric layers between high power dissipating devices and thermal ground, i.e., the base of a heat sink, is substantial. Thus power devices are frequently mounted on a heat spreader attached to the dielectric layer to reduce this thermal resistance. This paper provides analytically-derived formulae and accompanying dimensionless plots for the thermal resistance between the power device/heat spreader interface and thermal ground as a function of the relevant dimensionless geometric and thermal parameters for the axisymmetric and one-dimensional Cartesian geometries. These formulae/plots can be used to properly size the thickness and footprint of heat spreaders above dielectric layers. The utility of the results is demonstrated by an example.