Introduction <p>This study primarily aimed to compare the accuracy of acetabular cup positioning between the portable inertial navigation system (INS) Navbit Sprint and the manual method during total hip arthroplasty (THA) using the same cup in the supine position. The second aim was to investigate patient factors associated with cup positioning accuracy for THA with a portable INS.</p> Materials and methods <p>This study included 105 consecutive patients who underwent THA using a portable INS (n = 54) or manual methods (n = 51). The absolute difference between the target and postoperative angles, as determined by postoperative computed tomography data (placement error), was compared between the portable INS and manual method groups. The absolute value of the difference between the navigation and postoperative angles (measurement error) was also evaluated, and related factors were analyzed using multiple regression analysis.</p> Results <p>The average placement error with the portable INS group was 3.6 ± 2.7° and 2.6 ± 2.5° for inclination and anteversion, respectively. For the manual method group, the absolute difference was 6.0 ± 4.0° and 4.5 ± 3.5° for inclination and anteversion, respectively. Significant differences were observed between groups. The average measurement error of the portable INS group was 2.5 ± 1.9° and 2.7 ± 2.2° for inclination and anteversion, respectively. The measurement error for inclination significantly correlated with the leg length discrepancy.</p> Conclusions <p>Cup positioning with the Navbit Sprint was significantly more accurate than the manual method during THA using the same cup in the anterolateral supine position. However, a significant leg length discrepancy could negatively impact the accuracy of cup positioning inclination.</p>