This chapter describes a streamline curvature throughflow method which has been developed specially for radial machines, such as centrifugal compressor stages, pumps and radial turbines. The method can also be used for axial machines, both compressors and turbines, and for axial compressors with a centrifugal rear stage. The terms in the governing equation are described in some detail to illustrate how the geometry controls the pressure and flow gradients both along and normal to the streamlines in the compressor. Such codes have been in use since the 1960s as they a offer a good overview of the ideal velocity and pressure distributions through a component. They also highlight blade loading issues and identify risks of choking. In the design process, the throughflow method weeds out the weakest design options very quickly and thus eliminates the need for more complex, time-consuming CFD simulations on poor designs. A comparison of a throughflow calculation with the test results on the Eckardt impeller A shows the limits of the method and the efficacy of this procedure for preliminary design.