A model and procedure for calculating turbulent mixing of a confined jet is presented. The flow consists of a high velocity jet which is directed into a constant diameter duct, where it mixes with a concentric, low velocity stream. The flow model which includes the wall boundary layer covers the entire flow development from the mixing tube entrance to the start of fully developed flow conditions. The calculated results are presented for a range of flow conditions and are compared with the experimental results of a previous paper by the same authors. It was found that including the boundary layer, as part of the model, gave results which were significantly different and in better agreement with experiment for small and moderate velocity ratios.