Two Different Diameters
The diameter of the toilet flange will be either 3 or 4 inches, which is the diameter of the flange hole, not the diameter of the flange's outside perimeter. When you visit a plumbing website or a home improvement, hardware or plumbing store, the flanges will be in either one of these two diameters. The flanges with 3-inch diameters are designed to match the minimum diameter of toilet drainpipes, which is set at 3 inches by plumbing codes. However, while many homes do use a toilet drainpipe of three inches, others use a larger drain pipe with a diameter of 4 inches.
What the Flange Does
Without the flange, the toilet's drain outlet, which sits on top of the bathroom floor, has no solid connection to the drainpipe, which stops just below the floor. The flange provides this connection. The transition from the toilet drainpipe to the flange is made watertight by a wax ring, placed either on the bottom of the toilet or on top of the flange during toilet installation. The flange also mounts the toilet to the floor through the bolts that you find on either side of the toilet base, often underneath decorative caps.
Parts of a Flange
If you actually look at a flange when it's not installed on the floor, where you can normally only see the top of it, you notice that the flange has a ring on the top part of it and a bottom part that's shaped like a cylinder. The bottom portion is the part that mounts directly on top of the drainpipe or fits around it. It's called the hub. The ring on top is called the mounting flange, and you mount it on top of the floor.
Replacing a Flange
If the toilet rocks more like a children's ride than a stable fixture, the flange may be broken. If just one section of the flange is loose or broken, you can buy a toilet flange support kit or a replacement flange kit to support loose flanges. If a flange needs replacement, measure the diameter of the mounting flange hole using a tape measure, and use this measurement to obtain a flange of the same size. The bottom hub is often made of PVC while the top ring is usually metal. Flanges are also frequently made of ABS, brass, copper, stainless steel or other materials.