Various mathematical models and experimental methods have been developed for finding the effective orifice area in spool valves. The orifice equation for turbulent flow is often applied to spool valves to determine the volumetric flow rate of the fluid passing through the spool valve. This equation involves the discharge coefficient, C/sub d/, in calculating the effective area. The discharge coefficient has been to shown to vary with the spool position and Reynolds number. For small displacements of the spool, mathematical models have been developed to determine the discharge coefficient. However, for larger displacements, C/sub d/ is determined with greater uncertainty. An experimental procedure has been proposed to find the discharge coefficient of a spool valve as a function of the spool displacement and pressure drop across the orifice. This procedure involves measuring the flow rate through the valve at certain spool positions for stationary spool positions or time-varying spool positions. The physical area of the orifice created by the moving spool can be calculated from physical dimensions. Finally, pressure drop across the valve is measured while measuring fluid temperature. Flow rate, orifice area, pressure drop, and temperature are used to calculate the discharge coefficient from the orifice equation for turbulent flow.