Abstract: Automatic exposure control based on noise index has been commonly implemented in current medical X-ray CT scanners in order to minimise radiation dose to patients while maintaining image quality for patient diagnosis. Automatic peak voltage control is also desirable as it is an important scanning parameter that has significant impact on both image quality and radiation dose. In this paper, a general attenuation based governing equation for simultaneous automatic controls of both mA s and kV p is presented which was derived from the first principle of radiation dose and image quality optimisation. This equation can be applied separately for automatic mA s control only or automatic kV p control only. The equation for automatic mA s control only states that the mA s should be increased by 3.8% for every 1cm increase of the patient body's sectional thickness if the peak voltage is kept constant. For automatic kV p control only, the equation states that the peak voltage should be increased by 1.53% percent on average for every 1 cm increase of the patient's body sizes if the mA s is kept as a constant. For a constant body thickness, the equation states that the average percentage mA s reduction should be 2.49 times the percentage peak voltage increase. The implementation of these equations in medical CT scanners has been described in a recent Australian patent.