Standard Pipe Sizes for Toilets

Different plumbing pipes carry water to and from your toilet, and they don't all have the same diameter. If you plan to install your own toilet, you have to know which size pipe to use for each plumbing route. While there is some leeway in the size of the pipe used for your toilet, it is always best to stick with the standards for a smooth installation.

The drain pipe in your toilet carries waste away into the home's main drain line. The standard size for a toilet drain pipe is 3 inches in diameter, but drains up to 4 inches in diameter are sometimes used. Typically, 3-inch pipes carry the water to the toilet, but a 4-inch pipe may be used to move wastewater out of a house to a sewer or septic tank.

The drain pipe connects to the flange, which mounts the toilet itself, and this flange is usually built to connect to 3-inch, schedule 40 toilet drain pipe. In most homes, PVC drain pipe is used instead of metal pipe. Plumbers use piping-size tables to determine the size of the pipe to be needed.

The supply line to the toilet is standard 1/2-inch copper or plastic pipe routed from the main water line or a 1/2-inch cold water line. This standard is used throughout the home to supply faucets, showers and any other appliance requiring water, including the washing machine.

This 1/2-inch pipe connects to the home's 3/4-inch water pipes. The water supply line does not run directly to the toilet, however. A supply tube connects the supply pipe to the tank.

The closet bend is the elbow joint underneath the toilet that connects the vertical toilet drain pipe to the sloping drain pipe leading to the home's main drain stack. The closet bend actually has two diameters because it connects the 3-inch drain pipe to the 4-inch drain line.

The top of the closet bend is 3-inch-diameter pipe, while the other end is 4-inch. This pipe fitting is set under the toilet in the floor joists or ground (in toilets on concrete slabs).

Metal or plastic pipes are typically joined by a variety of fittings. These are designed to join pipes in a straight line, turn corners, branch in two directions, or connect to a fixture.

The drain line leading from the closet bend to the drain stack is 4 inches in diameter. This is a standard size pipe for toilet drains in most homes, and all main drain stacks usually consist of 4-inch-diameter pipe. However, some homes may have a smaller drain pipe diameter. In this case, a reducing connection attaches the closet bend to the main drain line.

Most homes, however, use a 4-inch drain or stack pipe. Four-inch pipes are also used to support multi-family units with up to 500 fixtures. Only large industrial buildings, apartment complexes or high rises have a need for larger-diameter drain pipes.