Abstract The flow analysis and design of jet pumps were until now mostly based purely on experiments and simple theories developed from energy and momentum balances. In this paper a more detailed analysis is presented, based on the Patankar—Spalding, finite-difference method which has been modified to predict two-dimensional axisymmetric jet pump flows. The solution is obtained by means of a marching-integration technique using an implicit scheme and integration over a micro-control volume. The analysis explains some details of the mixing between the two fluid streams involved, and facilitates determination of the optimum length of the mixing tube for a specified inlet condition. Predictions have been made and compared with experimental data pertaining to the centre-line velocity decay, wall static-pressure variation and axial velocity profiles for the mixing of both air and water streams, inside a constant-diameter duct. The theoretical predictions compare well with the experimental data and thus facilitate determination of the optimum mixing-tube length.