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.