Planimetric volumetry is used in clinical practice when accurate volume determination of the prostate is needed. The prostate volume is determined by discretization of the 3D prostate shape. The are of the prostate is calculated in consecutive ultrasonographic cross-sections. This area is multiplied by the distance between the cross-sections and the total volume is determined by summation of all contributions. Besides the quality of the automated outlining, the accuracy of this method depends on this intersection distance and on the angle of the scan plane with the probe. Also, the location of the first cross-section is of influence. This paper describes the influences of these parameters on the accuracy of the volume determination using a simple prostate model. This theoretical influence is compared to clinical volume determinations using automated planimetric volumetry with different step sizes. From our data, it is concluded that a step size of 4 mm for planimetric prostate volume determination is a good compromise between investigation time and accuracy in a clinical setting.