the present invention concerns a method of operating a wind power installation , wherein the wind power installation has an aerodynamic rotor in the form of a horizontal - axis rotor having a hub with at least one rotor blade , and provided on the rotor is at least one load measuring means for detecting a wind loading on the rotor , the method includes the steps : rotating the rotor of the wind power installation without or with a slight wind loading for calibration of the load measuring means and in that case recording a load measurement with the load measuring means , and calibrating the load measuring means based on the load measurement and previously known weight forces occurring at the rotor .