Toilet Drain Pipe Sizes – How to Choose The Right One?

Stuck with toilet drain pipe sizes? I wrote this definitive guide just for that kind of situation. Knowing the right toilet drain size is obligatory for the proper installation of this essential bathroom fixture. Additionally to the drainpipe size, I will tell you about the right sizing of the toilet supply line and the toilet closet bend.

What size toilet drain pipe should I use?
3-inch and 4-inch diameter pipes are both options for toilet drains in modern plumbing systems. The wider the pipe, the more waste it can carry and the less likely it will be to clog. A 4-inch pipe can carry twice the waste of a 3-inch pipe.

Actually, all the sizes of plumbing fixtures tick to the Uniform Plumbing Code, but differences in state code may occur, so it’s also highly recommended to contact your local building department and ask them to email you a plumbing code chart.

Toilet Drain Pipe

Waste line sizes in most modern buildings are 4 inches. This is a more universal size that allows almost 2x more waste per unit of time and is a lot less prone to clogging.

Many older buildings still have 3-inch main waste lines. Regardless of that, you can install both 3 and 4-inch toilet drain pipes. If a single drain pipe is purposed for more than one toilet, 4 inches is recommended.

Supply Line to the Toilet

Toilet water supply line size is, of course, much more compact than the waste line as the supply line pumps water into the tank slower. Depending on the placement of the toilet, the supply line pipe can be from 9 to 20 inches long, but it’s almost always ⅜-inches in diameter.

Drainage system installation

I recommend you install a flexible line as you can bend it to have the cleanest look possible and quit unnecessary measurements. The only thing you must ensure is that the length of the part will match the distance from the valve on the main water supply branch to the toilet’s water tank inlet.

Toilet Closet Bend

This elbow-shaped part is the last in the toilet drainage system, and it lets the exit flush change its direction at 90° and go horizontally to reach the main drain line.

Similar to the main toilet drain pipe, it can be 3 to 4 inches wide in order to match the drain pipe. Don’t forget that the toilet flange must also match the closet bend.

Toilet Flange Size

The standard toilet flange size is 4×3 inches. The top side connects to the toilet, while the bottom side matches the 3-inch schedule 40 pipes. If both the closet bend and the drain pipe are 3 inches in diameter, you have to use a 3-inch flange.

If you are not sure about the exact sizes of your toilet pipes, you can get a combined 3 4- inch flange that lets you attach 4-inch pipes and slide in 3-inch pipes.

Drain Line From the Toilet

While the toilet’s drain pipe can be 3-4 inches wide, the main drain line that locates the waste from the house to the septic tank or a sewer should be 4 inches to deal with wastewater from the toilet, shower, sink, washing machine, and other fixtures of your house or apartment.

Toilet Drain Pipe Size FAQ

Drain pipes in the basement

Do you have more questions? Make sure to skim through this section then!

Can a toilet drain be 2-inch?

I do not recommend using a 2-inch toilet drainage pipe size unless you are planning to install an ejector system. The recommended standard toilet drain pipe size is from 3 to 4 inches, and you’d better stick to that rule to avoid any flushing issues in the future.

What size PVC pipe is used for toilet drain?

Regardless of the pipe material and your local plumbing code, the drain pipe must be at least 3 inches to deal with the standard modern toilet that makes 1.6 GPF (gallons per flush). Toilets that use more than 1.6 GPF may require a 4-inch pipe.

How many toilets can you put on a 3-inch drain line?

A single modern toilet load on the drain line is usually equal to 3-4 DFUs (Drainage Fixture Units), while a 3-inch drain line can handle a maximum of 20 DFUs. Just don’t exceed this limit or install a 4-inch drain line if you have 2 bathrooms on one stack and they require a higher capacity.

Are all toilet drains the same size?

Standard plumbing pipe sizes for toilet drains are 3 and 4 inches, so, yes, toilet drains can be of different sizes.

Also read: 

How Big Are Toilet Pipes?

Now you know all the right answers! Make sure you match the toilet model you are going to install with the plumbing fixtures of your bathroom, install the thing, and forget about it for years! All you need is my simple instruction and understanding of plumbing basics.

What are your toilet model and its plumbing requirements? If you’re not clear on that, you’re welcome to get in touch with me for assistance via the comments section. You can also figure it out in the user manual or by contacting the manufacturer directly. For more useful info, guides, and product reviews, stay tuned to my blog!

Gerald Carpenter

My name is Gerald Carpenter and I am a professional plumber in the third generation. My father was a plumber, my grandfather was a plumber. I wish I had records on my other ancestors.

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