ESDU AERO 00.03.31 gives curves, based on a hypersonic similarity solution to the Prandtl-Meyer expansion relations that allow the static pressure difference across the expansion to be evaluated knowing only the upstream Mach number, gamma, and flow deflection angle. The equation for the curves is given. A sketch shows boundaries, in terms of the upstream Mach number and the ratio of flow deflection angle to the maximum, of errors of 1, 2, 3 and 4 per cent incurred using those curves compared with exact solutions. Another graph plots the maximum flow deflection angle against upstream Mach number from 1 to 10 and gamma from 1 (to aid interpolation) to 1.67 and the exact equation for those curves is given.