A toilet flange is a mounting base for your toilet that is installed on top of a soil pipe that connects to the closet bend, the angled pipe leading to your drain. The flange has a flat surface with bolt mounting slots onto which the toilet mounts, and it lays flat and flush to your floor. There are several sizes of flanges for different sized pipes, and, if you have not installed these flanges before, it's helpful to familiarize yourself with them.
Standard 4x3 Toilet Flanges
One common standard toilet flange size is 4x3, and this is the measurement you will find beside most flanges available at your hardware or plumbing stores. The 4x3 size refers to the two diameters in the flange pipe. The top side of the flange, which connects to the toilet, is 4 inches in diameter, while the bottom diameter of the pipe is 3 inches. The bottom is designed to fit on 3-inch schedule 40 pipe used in most homes as standard sized drain pipe. This is also the size of the closet bend pipe in most cases.
The 3-Inch Flanges
Many flanges are simply 3 inches in diameter, which means the top and bottom are each 3 inches wide. These flanges must install on a 3-inch-diameter schedule 40 drain pipe or directly to a 3-inch closet bend. When installing this type of flange, make sure your piping is sized properly. If it is not 3-inch pipe, consider using a different flange, because if you have to install a reducing bushing, this raises the position of the flange and may affect the overall alignment of the plumbing.
The 4-Inch and 3-Inch Flanges
Another type of flange installs on both 4-inch and 3-inch schedule 40 drain pipe. The flange is manufactured so it fits over the top of a 3-inch pipe (in other words, the pipe slides into the flange), or the flange fits inside a 4-inch pipe (the flange slides into the 4-inch pipe). This flange is appropriate if you are unsure of the size of your toilet pipes, or if you need to install a drain pipe between the flange and closet bend because you cannot directly connect the two.
Odd Sized Flanges
Other odd sized flanges exist. Typically the flanges are similar in size to the 4x3 flange, however. An ABS plastic flange with a 3 1/2-inch bottom diameter and a 7-inch top diameter is available. This flange is often used to connect an ABS drain pipe to the toilet. Extension kits to raise the height of the flange for new floor installations are also available. The flanges are the same diameter as standard flanges, and they simply extend the top of the flange upward to be flush with a new floor. An offset flange with a diagonal set opening is also available to provide additional space for an elongated toilet if a round toilet was previously installed.