We have investigated the accuracy of automatic calibration of digital pelvic radiographs using the single and double marker techniques. Both markers were applied by the radiographer at the time of routine, postoperative radiographs. Each radiograph was loaded into the TraumaCad(TM) software package which calculated the magnification and scaled the radiograph automatically. The median error of magnification in the double marker group was 1.14% when calculated automatically. The median error of the single marker was 5.98%. The relationship between true and predicted magnification was strongest in the double marker group with excellent correlation (r=0.91) and agreement (ICC<0.91). We believe that the double marker method with automatic calibration is the most reliable method of scaling a pelvic radiograph in clinical practice currently available.