An expression in the literature has been used to determine the spall strength of materials from measurements of the surface velocity on reflection of a nonsteady shock wave. It uses initial density, the sound speed, and the difference between the initial surface jump‐off velocity and the minimum in the velocity seen when the spall shock reaches the surface. Workers using the expression have found it necessary to choose the sound speed carefully in order to get reasonable results. It is shown that this is due to the inadequacy of the acoustic approximation used to represent the release isentrope. Approximation of the release isentrope by the Hugoniot makes it unnecessary to use special velocities and gives better agreement with experiment.