Soil heat flux transducer calibration, according to theory, is influenced by the thermal conductivity difference between the transducer and the calibration medium and the geometry of the transducer. This study was conducted to compare the influence of these parameters on the calibration factors of two types of commercial soil heat flux transducers with different material thermal conductivities and different geometries. A theoretical calibration equation was developed and evaluated. Calibrations of 14 transducers representing two commercial types were conducted in the laboratory using steady-state conductive methods over a range of heat fluxes from 40 W/m2 to 200 W/m2. The calibration medium was dry and saturated sand with a thermal conductivity varying from 0.3 to 3 W m−1°C−1. The mean calibration factor for one type of transducer was 12% lower than the mean manufacturer's calibration factor instead of the 26 to 36% lower value predicted by theory. The other type of transducer had a mean calibration factor 7% greater than the mean manufacturer's calibration factor in contrast to the 1 to 11% larger value predicted from theory. The computed geometric factors were 1.07 and 0.89 for the circular and square transducers, respectively. These factors were less than the theoretical value of 1.70 for each shape of transducer but similar to experimental values of 1.02 to 1.31 from previous studies reported in the literature. The thermal conductivity of the calibration medium and the geometry of the transducer affects the calibration factors of soil heat flux transducers, basically according to theory.