What Size Drain Pipe for a Toilet Do I Need?

Written by

Paulk Webb


Freddie J. Hagopian

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what size drain pipe for a toilet

Drain pipes play an integral part in moving the wastewater from the toilet to the septic tank. But the question is, “What size drain pipe for a toilet do you need?”

Toilet drain pipes are available in two common sizes – three inches and four inches. The 3-inch drain pipe is perfect for a small home. If you have multiple toilets at your residence, consider upgrading to a 4-inch one.

Let’s discuss more about drain pipe sizes below.

Common Drain Pipe Sizes


In modern plumbing pipe fixtures, you can either choose a 3 or 4” drain pipe for your toilet. Let’s differentiate the benefits and function of each toilet sewer pipe size, so that you’ll know which to pick.

  • 3 Inches

The standard toilet drain pipe size is 3 inches. This standard size code is enough to move wastewater for toilets that use no more than 1.6 gallons per flush in small houses. If you have more than one toilet at home, you’ll likely face clog problems.

You can connect this standard drain pipe to a 4-inch one through a closet bend if you want to improve waste removal performance. The 3-inch pipe often sits above the closet bend connecting directly to the toilet while the 4-inch pipe goes below directly to the septic tank.

  • 4 Inches

If you have multiple restrooms at home, then a 4-inch toilet waste pipe is a better option. The large capacity of this size can move more waste than the standard three-inch one. Even when two toilets flush at the same time, the huge pipe won’t clog easily.

Factors Influencing Drain Pipe Size Selection

Choosing the right drain pipe size can be easy if you know what you need. Here are some factors you need to consider when selecting the right size of a drain pipe.

  • Application


The drain pipe size varies according to its application. A toilet drainage pipe measures 3-4 inches while a bathroom drain pipe could either be 1.25 inches or 1.5 inches, depending on your local housing code. The standard toilet water supply line size is about ⅜ inches or 9.5 mm.

The bathtub drain size is typically around 2 inches, which is similar to shower, washing machine, and laundry drain pipes.

  • Drainage Fixture Unit (DFU)


DFU determines the possible amount of waste sent into the drainage pipe, consisting of different plumbing fixtures. For buildings, the minimum waste pipe size of toilets is 1.5 inches. 

One DFU equals one cubic foot of water carried to a 1-¼-inch pipe for one minute or approximately 0.47 liters per second. A 3-inch pipe can handle a total of 48 DFU in vertical and 35 DFU in a horizontal setup. The maximum length for a vertical toilet PVC pipe setup is 65 meters.

A 4-inch pipe can handle almost five times more, with 256 and 216 maximum units in vertical and horizontal positions respectively. You can also use a longer vertical pipe up to 91 meters.

  • Cost-Effectiveness


Installing a 3-inch drain pipe is more cost-effective than a 4-inch one if you have a single toilet at home. However, it becomes more practical to switch to a 4-inch pipe if you have multiple bathrooms in your residence. You can’t compromise a clog-free performance with price. 

Measuring And Determining Drain Pipe Size


Having the right fit is key to getting the best performance of your drain pipe. Fortunately, measuring the drain pipe size is easy. Here are the steps.

  1. Face the end of the pipe and get a measuring tape.
  2. Run the tape in a straight line from side to side through the center, including the pipe wall. This will give you the outside diameter.

Take note that your measurements might be more than the actual pipe size rating. If you get around 2.38 inches, choose a 2-inch drain pipe. A 4-inch pipe suits best for results that yield 4.5 inches.


Now that you know what size drain pipe for a toilet is proper, you can revamp your toilet or fix the drains with much ease. Make sure that you get the right size to avoid clogs in the future. Other than the size, make sure that you’re choosing a pipe that fits your application and budget.

Let us know your experiences with drain pipes in the comment section below. Feel free to share this article online if you like it.

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