For several years we have used Resonant Ultrasonic Spectroscopy (RUS) for routine determination of elastic-stiffness coefficients [1]. RUS has also been used elsewhere as reviewed by Maynard [2] and by Migliori [3]. Therefore, it is important experimentally establish the uncertainty of the RUS method. Previously, we have compared RUS with pulse-echo superposition and with other resonance methods [4,5] In this paper, we describe the process we used to determine that the RUS method has a 0.15% uncertainty, which is comparable with our best short-pulse time-of-flight measurement methods.