Three dimensional well block radius determiner machines, systems, program products, and computer implemented methods are provided to determine a three dimensional equivalent well block radius of a perforated grid block cell, with three dimensional flow, of a three dimensional coordinate grid block constructed over a three dimensional simulated well formation in a finite difference petroleum reservoir simulator. Various embodiments of the invention, for example, can beneficially account for both horizontal and vertical flow of oil through a well perforation without the need for complicated, expensive, and time-consuming numerical or iterative solutions. Embodiments of the present invention, for example, can be used as a part of legacy simulators thereby providing more accurate well block radius calculations, by accounting for both horizontal and vertical perforation flow, without introducing significant implementation hurdles, development costs, or simulation runtime delays.