To calculate the tibial wedge size in preoperative planning of high tibial osteotomy, the weight-bearing line (center femoral head to center tibiotalar joint) is first restored to a selected position on the lateral tibial plateau. Ten full-standing roentgenograms were examined and used to derive mathematical formulas for correcting limb alignment. A 3-4-mm shift in the weight-bearing line on the tibia occurred with each degree of tibiofemoral angulation. This showed that the position of the weight-bearing line is sensitive to the lengths of the tibia and femur as well as the surgeon's preoperative calculations. The problem of increased varus angulation due to slack lateral ligament restraints with distraction of the tibiofemoral joint was analyzed. Trigonometric analysis showed that each millimeter of lateral tibiofemoral joint separation caused about 1 degree varus angular deformity, requiring subtraction in preoperative calculations to avoid overcorrection. An algorithm was designed to evaluate complex lower-limb alignments.