A two-dimensional finite-difference time-domain method with uniaxial perfectly matched layers was applied to analysis of a three-layer wide-aperture microwave calorimeter. The method was shown to be an efficient tool for calculating the optimum geometric parameters of a semi-infinite model dummy load. Using the time-domain results, the frequency-dependent transmission/reflection coefficients of the load were calculated for normal incidence of a monochromatic plane wave at operating frequencies range of 1–12 GHz for the first time.