A novel self-consistent approach for the determination of the correction factor of a waveguide microcalorimeter is described. The method is based on the measurement of offset shorts of different length followed by one single calibration measurement of an unknown thermistor device under test (DUT). Without precisely measuring the radio-frequency (RF) power incident to the reference plane, both the effective efficiency of the DUT and the calorimeter correction factor are determined. The method theoretically takes into account the different surface current distributions and associated waveguide losses being existent for offset short, as well as for the DUT measurements. It has been applied to a twin-type microcalorimeter (33–50 GHz) that has recently been developed at Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany, which results in low uncertainties of the measured effective efficiency of thermistor power sensors.