A method of automatically calibrating a motion detector and a motion detector adapted for calibration using the method. The method comprises setting the motion detector to a calibration mode, waiting a preset period of time for the installer to walk to the furthest distance, receiving a Doppler signal generated by motion at the furthest distance from the microwave motion detector within the protected area, measuring an amplitude of the Doppler signal and storing a motion detection threshold for the microwave motion detector. The motion detection threshold is a proportional to the measured amplitude.