The success of removal of suspended solid from suspending liquid by a hydrocyc¬lone is a function of the dimensions of the hydrocyclone, operation variables, and physical properties of the liquid and particles. Hydrocyclone develops separational power through the use of fluid pressure energy. There is thus a loss in pressure or a pressure drop, across the unit which is an important operating variable. In this article the pressure drop in the hydrocyclone is given by an equation which depends on the velocity head, Reynolds number, and shape factors of the hydrocyclone. A summary of the utility of the equations presented by earlier authors is also given.