A method of in situ calibrating load sensors of a horizontal axis wind turbine is described. The method comprises the steps of: a) determining a rotor azimuth angle of a first wind turbine blade, b) determining a pitch angle of the first wind turbine blade, c) measuring loads in a first cross-section of the first wind turbine blade using the first load sensors, d) calculating theoretical loads based on at least the rotor azimuth angle and the pitch angle of the blade determined in steps a) and b), e) comparing the loads measured in step c) with the theoretical loads calculated in step d), and f) calibrating the first load sensors based on the comparison of step e), wherein the calibration are based only on measurements carried out, when the generator is cut out.