The effective orifice area (EOA) of a mechanical heart valve is an index of how well the valve design utilizes its primary orifice area (POA). In vitro measurements of EOA of aortic valves were maintained by means of pressure drop and root mean square flow rate measurements in a pulse duplicator during systole. Edwards-Duromedics, St. Jude Medical, and Carbomedics aortic valves of sizes 19 19 mm, 21 mm, 25 mm, and 27 mm were analyzed over a cardiac output range of 3 to 7 liters/min. The resultant ratios of EOA/POA were in the range of 0.6-0.8. A simplified equation suggested by the FDA was used in this study to calculate EOAs. To agree with original assumptions of the simplification, the entrance flow area (EFA) where upstream pressure is measured, must be large as compared to that of the test valves. If not, the formula can yield questionable results such as implying that the EOA can be larger than the POA (Walker P et al, 1992) [1]. This paper discusses the limitations in using such an equation. In conclusion, we suggest utilizing the parameter square root of 1-(POA/EFA)2 to evaluate the validity of the data processing, before using the equation. The parameter should be close to one, and in this study it was 0.997.