This paper describes the calibration of an optical particle counter with the help of an aerodynamic particle sizer. The calibration method and influencing errors are discussed. The transfer characteristic of the optical particle counter is determined with monodisperse fractions of polystyrene latex spheres and verified with spherical particles (glycerine) of known refractive index. In calibration experiments with three optical particle counters of the same type the method proved to be transferable to different devices. The described method was validated for nonspherical particles (quartz). The results from calibration by means of an aerodynamic particle sizer are in perfect agreement with the results from calibration by means of sampling cyclones. The enhancement of scattered light intensity due to irregular particle shape is demonstrated by the comparison of experimental quartz calibrations with calculations for spherical quartz particles.