Ultrasonic clamp-on flow meters operating with the transit time method use an acoustical calibration factor, which determines the transmission angle in the fluid. This can be measured indirectly by a flow calibration. The result of a flow calibration, however, depends on parameters such as the pipe diameter and the flow profile that are not part of the clamp-on meter being calibrated. A new method is presented that allows for direct determination of this acoustical calibration factor. With this method, transducers can be calibrated for a designated flow range and pipe range independent of calibrated master flow meters, and respective pipes and calibration facilities.