A theory is outlined by means of which it is possible to design ducted fans directly and without having to choose any parameters in an arbitrary fashion. Optimum sizes and fan speeds are theoretically determined. The design of two fans, one having inlet guide vanes and the other of the low slip-stream-rotation type having NPL straighteners and often used in wind tunnels, are considered by way of example. While achieving a considerable degree of rationality, the theory introduces a concept of annulus efficiency that is still of an empirical nature. The empiricism is due to a present lack of knowledge about three-dimensional fan losses, and suggests further investigation.