3" hub/inside 4" ABS, flush to floor, knockout closet flange will not break apart and cannot fall down drain pipe. Strong, corrosion-resistant 1-piece stainless steel ring eliminates cracked flanges. Easier installation, no need to hold knockout while hammering. Holds 5 floors (22 P.S.I.) for head test. The inside diameter of pipe can vary significantly. These flanges are designed to fit inside most, but may not fit all types of pipe.