Background <p>Accurate intra-operative assessment of knee extension angle is essential in total knee arthroplasty (TKA), as residual flexion contracture or hyperextension is associated with inferior post-operative function and increased implant wear. Visual estimation remains widely used, but prior research has demonstrated substantial inaccuracy. This study evaluated the accuracy and reliability of a digital inclinometer technique for measuring knee extension angle during TKA, using computer navigation as the reference standard.</p> Methods <p>A prospective comparative study was performed on 130 knees undergoing primary posterior-stabilized TKA. After trial implant insertion, knee extension was assessed using two methods: visual estimation and a digital inclinometer method utilizing the femoral chisel holder and anterior border of the tibial crest. All measurements were compared with OrthoPilot navigation. Accuracy was defined as measurements within ± 2° of navigation. Correlation, agreement, and intra- and inter-observer reliability were evaluated.</p> Results <p>Visual estimation underestimated extension and showed a mean difference of − 3.5° ± 4.1° versus navigation (<i>p</i> &lt; 0.001). The proportion of knees within ± 2° of navigation was only 26.2%. In comparison, the digital inclinometer demonstrated a significantly smaller mean difference of 0.1° ± 1.6° (<i>p</i> = 0.753), with 86.9% of knees meeting the ± 2° accuracy threshold. Superiority of the inclinometer persisted across all subgroups of pre-operative deformity. Correlation with navigation was weak for visual estimation (<i>r</i> = 0.320; <i>r</i>² = 0.102), but very strong for the inclinometer technique (<i>r</i> = 0.894; <i>r</i>² = 0.799; both <i>p</i> &lt; 0.001). Bland–Altman analysis demonstrated minimal bias and no proportional error for the inclinometer, whereas visual estimation showed wide limits of agreement. Intra- and inter-observer reliability for inclinometer measurements was excellent (ICCs of 0.91 and 0.93), superior to visual estimation.</p> Conclusion <p>The digital inclinometer technique provides accurate and reproducible intra-operative knee extension measurements, outperforming visual estimation and approximating navigation-based assessment. This low-cost approach is feasible where navigation is unavailable.</p> Trial registration <p>Thai Clinical Trials Registry (TCTR20180828002). Registered on 28 August 2018. Prospectively registered.</p>