The commonly used standard goniometer has been shown to underestimate knee flexion. Computer-assisted navigation for total knee arthroplasty offers itself as an alternative method to quantify knee flexion. The goal of our study was to determine the reliability of each instrument in measuring intraoperative range of motion during total knee arthroplasty. We prospectively performed intraoperative measurements using both methods on 99 knees. We assessed the variability between the 2 devices and the confounding effect of body mass index. The mean difference between the goniometric and navigation measurements was significant for flexion, extension, and range of motion. Increasing body mass index accentuated this difference. The goniometric method underestimated flexion measurements as compared to navigation, especially in patients with high body mass index. Our study confirms that navigation is a reliable tool for performing in vivo assessment of range of motion.