A calibration methodology developed for custom-made heat flux sensors used for the investigation of hydrothermal spallation drilling is presented. An air jet stagnation convection calibration system allowing independent control of jet velocity and temperature, cooling water flow and temperature, stand off distance, and nozzle diameter was built. A thermopile device was used as reference heat flux sensor. Air jets with velocities up to 400 m/s, exit temperatures of 760 degrees C, and heat fluxes up to 0.6 MW/m(2) can be reached. The standard deviation of a reference heat flux measurement is below 2%, and overall uncertainty achieved is less than 5.5%.