How many Toilets can be on a 3-inch drain?

Are you planning to install a 3-inch drain pipe on your drain pipe, and are not sure what plumbing codes state about the number of toilets you can install on a 3-inch drain? Then, you are at the right place.

According to the International Plumbing Code (IPC), a drain pipe with a 3-inch diameter can handle a maximum of 20 drain fixture units (DFUs). A toilet group with a closet and shower head has a value of about 5 DFUs. However, with other water fixtures like bathtubs, and sinks, you can install a maximum of 3 toilets on a 3-inch drain line.   

Now, you might be confused about what these drain fixture units are, and how this value is determined for each water-consuming equipment. We will cover all these things, and I will answer some further questions regarding 3-inch drain pipes.

What is a Drain Fixture Unit (DFU)?

In plumbing, we have a unit defined for each fixture, that is based on the amount of discharge, frequency of its use, and time of its use. This unit which is a measure of hydraulic load a fixture puts on the drain line is called a Drain Fixture Unit or DFU.

Mathematically, one DFU is defined as 1 cubic foot of water discharged through a pipe of 1.25 inches in diameter for 1 minute.    

People often think of it as a discharge flow rate unit. However, it’s a design factor that plumbers take into account before designing a plumbing system.

Drain Fixture Units of Common Household Utilities?

The International Plumbing Code (IPC), which is mostly followed throughout America, has defined DFUs for all household fixtures. The drain fixture units of some of these bathroom accessories are illustrated in the table below:

Bathtub (with/without showerhead)2
Bar sink1
Water Closet (1.6 GPF)3
Kitchen Sink2
Bathroom Group (Water Closet + Showerhead + bathtub)5
Bathroom Group (Water Closet + Showerhead + sink)6

You can see that a bathtub has a value of 2 drain fixture units, while a sink and bidet each have 1 DFU. The toilet alone carries 3 DFUs. But, you know that a bathroom does not contain a toilet only, it has all other accessories including a bathtub, sink, and shower.  So, a full bathroom group makes 6 DFUs.

How many toilets can be installed on a 3-inch Drain? (With Calculations)

Now that you have known the drain fixture units of most bathroom utilities, now it will be easy for you to figure out how many toilets can be attached to a 3-inch drain.

As per International Plumbing Code, a 3-inch drain pipe can handle 20 DFUs. If you are to attach only toilets, you may add up to 6 toilets, based on the calculations. However, practically that is not feasible.

A toilet has a bidet along with it. Then, your bathroom also has a sink, showerhead, and bathtub. These collectively make up 6 DFUs. So, practically you can install up to a total of 3 toilets on the 3-inch drain line.

What happens if you install multiple toilets on a 3-inch drain?

As I mentioned earlier, a 3-inch drain can only handle up to 3 toilets. But what happens if you install more than 3 toilets on this drain?

First, DFUs are defined by the IPC and your local community strictly adheres to them. Installing more toilets means you are breaking the community laws for which you may have to face the consequences.

Second, more toilets on a drain pipe mean more waste load on the pipe. If the toilets are simultaneously used often, the drainage system will get clogged.

How many fixtures can a 4-inch drain handle?

a 4-inch drainpipe installed for bathroom
Image from Self Build Blog

Like a 3-inch pipe that can handle 20 DFUs, a 4-inch can handle 160 DFUs. It means that it can handle 8 times more fixtures than a 3-inch pipe. So, if a 3-inch pipe can handle 3 toilets, the 4-inch toilet can handle up to 24 toilets.

You can see how a one-inch difference in the diameter drastically changes the discharge handling capacity of the drain pipe. Since a 4-inch pipe can carry more waste than a 3-inch one and can handle a large number of water fixtures, it is commonly used in commercial buildings.


Now, let’s answer some more questions asked by the people regarding the installation of toilets.

Can two toilets have the same drain pipe?

Yes, it is completely normal for two toilets to share the same drain line. However, they do need to have different drain holes.

Which size drain pipes are commonly used?

Nowadays, drain pipes are normally available in two standard sizes, 3-inches, and 4-inches. A 3-inch pipe is normally used for domestic purposes while 4 inches is for commercial uses.

Which size drain pipe is better, 3-inch or 4-inch?

The standard size for a building’s sewer line in the present day is 4 inches. This is a more general size that permits roughly two times as much garbage to be collected in the same amount of time and is far less likely to become clogged.

Despite the advent of larger lines, many older structures continue to use primary waste lines that are just 3 inches in diameter. Whatever the case may be, both 3-inch and 4-inch drain pipes can be used in a standard toilet. If more than one toilet is going to be connected to the same drain pipe, then you should go for a diameter of 4 inches.

How many toilets can you attach to a 4-inch drain line?

As per IPC, a 4-inch drain pipe has the ability to 160 drain fixture units. This is 8 times the units a 3-inch pipe can handle. That means, roughly you can attach 24 toilets to a 4-inch drain.

Final Words

If you are planning to install a 3-inch drain pipe in your house. That means you can have a maximum of 3 bathrooms or toilets. However, if you think that a 3-inch won’t be enough to handle your needs, a 4-inch pipe is always another option to consider.

I hope that this post would have helped you make an informed decision. If you have any questions, let us know in the comments section below.

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