Purpose. To describe a reliable method of predicting native joint line and posterior condylar offset (PCO) using true lateral digital radiographs of the distal femur. Methods. PCO was measured relative to a line drawn parallel to the posterior cortex of the distal femur and the joint line was measured from the posterior condylar flare to the articular surface. A ratio was then calculated for these measurements relative to the width of the femur at the level of the flare. Two independent observers measured PCO and joint line ratio for 105 radiographs of the different knees and one repeated these measurements after one week. Results. There was a significant correlation between the width of the femoral diaphysis at the level of the posterior condylar flare with joint line (P = 0.008) and PCO (P = 0.003). Joint line and PCO could be predicted within 4 mm and 2 mm, respectively, using the identified ratio between the width of the femoral diaphysis at the level of the posterior condylar flare with measured joint line and PCO. The inter- (P < 0.001) and intra- (P < 0.001) observer reliability for these ratios were high. Conclusion. These ratios could be used to predict the native joint line and PCO.