Toilet flanges install on your bathroom floor, right beneath the toilet. The flange has a flat edge with a bolt insert that mounts the toilet in place and keeps it from rocking back and forth. The toilet flange usually has two diameters, one for the toilet opening and the other for the drain pipe it connects to.
Typical toilet flanges measure 4×3, meaning they have one diameter that measures 4 inches and another that measures 3 inches. The top section of the flange that connects to your toilet has a 4 inch diameter while the outlet below this has a 3 inch diameter.
That is so the flange mounts to standard 3-inch drain pipe (schedule 40) used in almost every residential home today. A few other flange sizes exist as well.
Some flanges just have one diameter, typically 3 inches. This is the size of the flange opening and outlet. There really is no advantage to this size, unless your toilet has a three inch drain hole opening in the bottom. It is difficult to connect a 3-inch toilet to a 4-inch opening.
Some, more versatile flanges include both a 4 inch and 3 inch connection option. The flanges slide over 3 inch pipe and into 4 inch pipe, making them ideal for contractors that encounter both sizes of toilet piping in a home. Toilet flange sizes cover the range as well. Some come in 3 1/2 inch diameters with a 7 inch opening.
The toilet flange size must fit the pipe it connects to without the use of reducing bushings, because far too often there is not enough room to install a bushing between the flange and closet bend.