because all of the mold and material factors are the same for n. if a cylinder is chosen for the geometry of the riser and the height to diameter ratio is locked, then the equation can be solved for a diameter, which makes this method a simple way to calculate the minimum size for a riser. note that if a top riser is used the surface area that is shared between the riser and the casting should be subtracted from the area on the casting and the riser.