Introduction: In high tibial osteotomies (HTO) the correction needs to be precise and intra-operative assessment is essential. The purpose of this study was to evaluate the use of the intra-operative clinical mechanical axis measurement and compare it to the post-operative weight bearing situation on standing whole leg radiographs (WLR). Secondly, we evaluated the preoperative planned wedge size and compared it to the final results in 27 HTOs. Methods: The mechanical axis deviation (MAD), expressed as a percentage of the tibial width (0% is medial edge, 100% is lateral edge) and the hip–knee–ankle angle (HKA) were calculated and analyzed. Preoperative planning was done by projecting the desired mechanical axis on the lateral tibial spine on the WLR and subsequently calculating the necessary correction angle. A 3° valgus correction was desired. Results: The results showed a preoperative standing MAD of 26.6%, or about halfway the medial tibial plateau. The MAD was corrected to 57.0% intra-operative, meaning a slight valgus. On the postoperative standing WLR, the MAD had shifted significantly to 62.0% (95% CI = − 9.2 to − 2.7; p < 0.01) compared to the intra-operative 57.0% axis. The HKA angle changed 8.2° from 5.1° varus preoperative to 3.1° valgus postoperative. Conclusion: When aiming the mechanical axis at the lateral tibial spine, a slight undercorrection was introduced, but due to a valgus shift while weight bearing, a satisfying final outcome of 3° valgus average was achieved. Preoperative calculation of the wedge size remains important for surgical planning, but it does not always correspond to the actual used wedge size.