The effectiveness of a computational fluid dynamics (CFD) commercial code to accurately predict the leakage rate for a windback seal was evaluated. The windback seal under consideration has a rectangular cavity and is similar in design to a gas tooth on stator annular labyrinth seal. The main difference is the windback seal has only one tooth, which continuously winds around the shaft like a screw thread. These seals are used in gas compressors to isolate the gas face seal from bearing oil. A purge gas is passed through the seal into the bearing housing. The helical design allows the seal to clear itself of any oil contamination. The objective is to determine if CFD simulations can be used along with a few experimental tests to study windback seals of this design. Comparison of measurement and predictions for a simple rectangular cavity windback seal shows predictions and measurements comparing very well with maximum differences of 5% for leakage rate. The variation of leakage with shaft speed and pressure ratio across the seal is accurately predicted by the CFD simulations.