toilet drain size

Toilet Drain Sizes

When you’re working with plumbing, you want to be sure that you know the size of the piping. That goes especially for installing bathroom fixtures like toilets and bathtubs.

A toilet drain pipe for most modern homes will be 3 inches in diameter, but there are a few models that can be up to 4 inches.

Here’s everything you need to know.

What Is The Standard Toilet Drain Size?

A 3 inch drain is the standard size for most toilets. By standardizing this piece of plumbing, it becomes much easier to repair or replace malfunctioning toilet drain piping.

It also makes it easier to be sure that your toilet model will be compatible with the plumbing in your home.

Are There Different Size Toilet Drains?

While the standard setup for toilet drains is 3 inches, there are sizes of up to 4 inches. In some cases, a 4 inch pipe might be used to maneuver the waste to the septic system or city plumbing system, while a 3 inch pipe is attached to the toilet itself.

The difference in size allows for more waste to be carried. When a drain line is 4 inches in diameter, it can move about twice as much waste, making it good for industrial applications.

On top of this, it’s harder to clog a 4 inch drain than a narrower one.

If two toilets are being flushed through the same drain pipe, you must use 4 inches. This will prevent potential system backups and clogging.

The minimum size for the drain diameter is 3 inches. You cannot have any pipe size narrower than that, because it will not be big enough to move the waste without clogging.

How Is Toilet Drain Diameter Measured?

The drain diameter is measured by the pipe attached to the toilet. This pipe siphons waste away from the toilet through the sewer pipe and into the septic tank or plumbing system.

You measure the opening of the pipe across the widest point of the mouth. That gives you the diameter.

If you’re not sure whether you have the right size, you can ask a plumber or a hardware store employee to double check that you’re working with the correct model.

How Long Is The Toilet Drain?

Toilet drains don’t have a standard length, though their width is standardized. The length of the pipe will depend on a few factors:

  • How far the toilet is from the wall
  • How much of the pipe fits into the toilet itself
  • How deep the pipe runs into the wall

The drain pipe should be long enough to connect to the rest of your household plumbing. If you’re working with a 4 inch pipe, you can thread it up to 10 feet before you need to connect it to the stack.

How Do You Choose A Toilet Drain?


Your toilet drain pipe should be a standard size. If it’s 4 inches rather than 3, you’ll be able to move more waste.

When multiple toilets are connected to just one pipe, you definitely want a wider size.

Otherwise, the best drain pipe configuration is best left to your plumber. As long as the materials are able to move water and waste without clogging or corroding, you shouldn’t run into any trouble.

How High Should The Toilet Drain Be?

The height of the drain will depend on your interior plumbing.

You should make sure that the flange is installed at an ideal height. The best height is just 0.25 inches above your bathroom floor.

Installing the flange in this area lets you seal the toilet with any kind of wax.


The standard size for a toilet drain pipe is 3 inches, but sometimes they can be up to 4 instead.

There are many aspects of installing a toilet and toilet plumbing that have nothing to do with the drain size. These will affect the best toilet model for you.