Effective Orifice Area (EOA) is a valuable diagnostic parameter for the determination of the severity of Aortic Stenosis(AS). A 4D Flow based measurement of this parameter was developed, and tested. The method was validated on a rigid phantom, applied to 4 healthy volunteers and 17 patient subjects with moderate to severe AS. The resultant EOA, for patients, was compared to their transthoracic echocardiography (TTE) with Doppler result. The comparison used Bland Altman analysis and linear regression which revealed a bias of 0.04 cm2 and correlation coefficient of 0.92. Showing a high level of concordance between the two modalities.