A method for quantitatively measuring internal defects in an as-cast steel product includes optically scanning at least a portion of a surface of the steel product with a scanning device to create a digital image thereof. The method further includes thresholding the image in a thresholding engine to isolate internal defects within the image and analyzing the thresholded image to determine an internal defect area, wherein the internal defect area includes an area of at least a portion of the thresholded image that is occupied by the internal defects. The method further includes determining an overall area of the portion of the thresholded image, calculating a fraction of the internal defect area relative to the overall area, and calculating an equivalent Mannesmann scale rating of the internal defects in the steel product based on the fraction.