A modification to accommodate loss is made to a method developed by D. Murray and E. A. Ash [1977UltrasonicsSymposiumProceedings, IEEE Cat. No. 77CH1264‐1SU (IEEE, New York, 1977), p. 823] for computing the surface acoustic wave slowness surface of piezoelectric substrates. The resulting algorithm is used to determine the slowness surface for the leaky or pseudosurface wave which propagates on [100]‐cut GaAs and for a cut 5° off from this. The data thus obtained are used in a diffraction theory computer program to predict the shape of beam profiles as they propagate in the substrate material and accurate predictions of beam shapes are obtained.