The appropriate correction factors have been determined to enable plane-parallel ionization chambers to be calibrated using a ⁶⁰Co photon beam with a known air kerma rate. These factors refer to different calibration conditions and to different types of plane-parallel chambers. The main purpose of these correction factors is to allow calibration procedures more widely practicable than those based on the use of an electron beam of sufficiently high energy. The condition required to apply the correction factors is that the characteristics of the chambers to be calibrated are the same as those reported for the chambers considered in the investigation. To this end the types of chambers investigated were among the most widely used plane-parallel chambers commercially available.