Calibration method for a power screwdriver as heavy-duty screwdriver, having a controllable drive motor and a downstream reduction gear, the working range of which is predetermined for predefinable tightening torques by a predefined motor output and by a predefined gear reduction, wherein a motor control unit has an input unit for the external specification of a particular tightening torque as setpoint input torque (M soll,ext), and wherein, for the calibration operation, the power screwdriver is connected by the gear output shaft to a measuring shaft of a precisely calibrated torque measuring device which senses the measured data as actual torque values (M ist,mess), characterized in that an electronic calibration unit is connected by data lines (8, 9) to the torque measuring device and the motor control unit and has an actuable programming and display unit, in that corrected, internal setpoint control values (M soll,korr) that are each assigned to predetermined setpoint input torques (M soll,ext) are able to be stored in the motor control unit, said internal setpoint control values being used, after calibration, to internally control the power screwdriver in operational use, in that, to prepare a calibration operation, power screwdriver data are input at the programming and display unit, and also a number of manually predefinable tightening torques that are spaced apart in the working range are predetermined and input as setpoint input torques (M soll,ext), in that, during the calibration measurements, the setpoint input torques (M soll,ext) are used as not yet corrected setpoint control values (M soll,korr) in the motor control unit, in that, after the start of a calibration operation, the predefined calibration measurements are carried out in succession, wherein, for each setpoint input torque (M soll,ext), an associated torque is measured as actual tightening torque (M ist,gemessen) by the torque measuring device, and in that, from value pairs identified in this way, respective supporting points (S 1 -S 5) are formed and stored in the calibration unit, in that, after the calibration measurements, a continuously extending measurement curve interpolated for intermediate values or a corresponding characteristic map is formed by the identified supporting points (S 1 -S 5) in the calibration unit, such that a clear assignment between a setpoint input torque (M soll,ext) and an actual tightening torque (M ist,gemessen) is present even for intermediate values, in that, by means of the calibration unit, the measurement curve or a corresponding characteristic map of the motor control unit is transmitted and stored there, and internal, corrected setpoint control values (M soll,korr) from the measurement curve are present, which can be used for motor control during operation after the calibration, in that, at the end of the calibration operation, the motor control unit is switched such that, for the further operational use of the power screwdriver, the measured actual tightening torques (M ist,gemessen) from the measurement curve or from a characteristic map are used as new setpoint input torques (M soll,ext) that are able to be input and the setpoint input torques (M soll,ext) used originally before and for the calibration are used as assigned corrected setpoint control values (M soll,korr).