Nomograms derived from mathematical analysis indicate that the level of malunion is the most important determinant of changes in the moment arm of the knee, the plane of the ankle and alterations in limb length. Testing in five patients undergoing reconstruction showed a mean error of postoperative limb length of 2.2 mm (sd 0.8 mm), knee moment arm of 4.7 mm (sd 3.3 mm) and ankle angle of 2.6° (sd 2.3°). These nomograms provide the information required when assessing whether a particular degree of angulation may be accepted.