This study presents newly developed charts to aid in early design of impulse turbine for wave energy extraction. These charts, based on the available experimental data, represent a simple approach to the performance evaluation of the turbine. The novel approach is applied in a case study that considers the optimum diameter design selection of next-generation impulse turbine power take-off. This allowed the selection of the correct impulse turbine sizing for a required rated power. The result is consistent for such an application, where the optimum rotor diameter would be 1.6 m for a maximum rated power of 400 kW.