A compressible flow jet pump theory is evolved which can be more easily interpreted for design purposes than existing theories. It consists of a one-dimensional analysis based on the momentum equation and on complete mixing, used in conjunction with an over-all pressure recovery factor which is found experimentally. Tests on a small jet pump are described and from the results it is shown that the overall pressure recovery factor can be related to a single parameter in the analysis for all operating conditions of each general pump configuration. In this way a general method for the correlation of results and for the design of jet pumps is established.