toilet drain pipe size

What Is The Standard Toilet Drain Pipe Size

Are you curious about what the proper toilet drain pipe size is? You’re fortunate since this piece highlights all there’s to know about toilet drain pipes, their sizes, and other frequently asked questions regarding the toilet plumbing network. 

In contemporary plumbing fixtures, sewage drain pipelines for toilets are either 3 inches or 4 inches wide. The larger the pipe, the more the sewer it can handle. Additionally, wider toilet drain pipes are less likely to clog. 

A 4-inch conduit can convey almost twice as much garbage as a 3-inch pipe, besides being able to handle sewer from multiple restrooms. 

Read on to find out more. 

What Size PVC Pipe Is Used For Toilet Drain?

In most households, plumbers pipe toilets with three-inch pipes. In contrast, the four inched ones become building drains beneath floors or unfinished basements to convey all wastewater from a residence to a septic tank or sewer. 

You can also employ four-inch pipes in capturing at least two bathrooms. 

How To Install Toilet Drain Pipes

You can fix toilet drain pipes individually. However, it’s advisable to have sufficient plumbing knowledge because installation errors can lead to health concerns from sewer gas leaks. If you are not sure about the procedure, contact a certified plumber.

Before beginning, verify with the local authorities about construction rules and regulatory prerequisites. Ensure that the dimensions of the drain pipe and that of the vent comply with the regulations. 

Furthermore, plan your task to incorporate any necessary inspections. 

You will need a 4-inch PVC pipe, wye fitting, PVC cement, PVC threaded adapter, reciprocating saw, screwdriver, closet flange, 3-inch PVC pipe, closet elbow, sanitary tee, 4- by 2-inch plastic reducing coupling, pipe cutter, 2-inch PVC pipe, 1/2-inch copper pipe, and fittings. 

After ensuring everything is in place, follow the guide below to install toilet drain pipes:

1) Locate Toilet And Drain Pipes

      a) Establish Access To The Pipes

Locate the primary drainage stack in the crawlspace or basement and mark where it runs up through the apartment and where it emerges on the roof. Determine the locations of the additional waste drain pipes and check for venting pipes. 

Assess the spot you can attach the restroom to the drain lines, vent pipes, and the stack. 

       b) Plan the Pipe Location

Make a precise strategy for running the toilet drain pipes. If required, remove the flooring and open up the walls.

Since your toilet boasts an inbuilt trap, you may design a direct drain route with as few curves as workable.

       c) Check The Sanitary Tee Space

Ensure you have adequate space to attach the sanitary tee to the combination fitting to a horizontal outlet or the main stack. 

     d) Confirm The Building Codes

Verify your building regulations and check for any restrictions on how extensive your toilet drain line can be before venting and attaching to the stack.

Read also Can a sink and toilet share the same drain

2) Cut And Assemble Drain Pipes

     a) Pipe Assembly Without Glue 

Snip the piping and configure them without gluing to ensure they are in the required lengths and suit the designated area. Ensure you have the necessary fittings to connect your lines to the existing vent pipes and drain. 

     b) Examine the Stack for Proper Support

Ascertain that you reinforce your existing drain or stack securely above and below the proposed placement of your connection. If applicable, integrate supporting braces, then cut a portion of the piping equivalent to the combination fitting or sanitary tee.

3) Glue Toilet Drain Pipes

          a) Use Glue And A Pipe Cleaner

Add piping cleaner and pipe glue to the fitting and the flange; then install the fitting, and bend it firmly into the flange. Do this after establishing that the existing drainpipes embody adequate flexibility to accommodate the new fitting. 

If you cannot shift the drain pipes or the stack, buy a no-hub flexible coupling and cut off the extra length of the old pipe equivalent to the coupling’s dimensions. Glue the extra pipe into the fitting’s upstream flange. 

      b) Glue The Fittings And Install The Coupling

Attach the coupling to the old pipe upstream, then fix the fitting to the downstream conduit using glue. Tighten the connection by sliding it over the joint. 

       c) Complete The Gluing Process

Proceeding from the primary sewer pipe back to the restroom, glue the remaining sections of the fittings and the conduit into place. Incorporate pipe straps every four feet to reinforce the piping. 

Bolster the last upward bend beneath the toilet and ensure it reaches a height high enough to allow the fixing of a toilet flange. 

Read also How many toilets on a 3 inch drain


If you’re attaching a newer toilet to an old stack, you solely need to run waste and supply pipes. A distance of approximately 10 feet between your restroom and the stack will facilitate this. 

A waste line that leads straight to the sewer would necessitate waste line venting. You can connect the vent pipe to the one running through the roof.

Should The Toilet Drain Be 3 or 4-inch?

The minimal toilet outlet size is 3 inches, irrespective of local plumbing regulations. The Drainage Fixture Unit (DFU) rating for a water closet discharging 1.6 gallons (sometimes less) is 3. 

Some older restrooms that flush over 1.6 gallons every flush are graded at 4 DFUs. 

What Size Is The Main Drain Line?

The main sewage line for a residential house is usually 4 inches wide, subject to local plumbing requirements. 

Read also 3 or 4 inches drain  for toilet?

FAQs on Toilet Drain Pipe Size

1. Are All Toilet Waste Pipes The Same Size?

Toilet drain pipes in the present plumbing measure 3 or 4 inches wide. The larger the piping, the less probable a fault will arise. 

A 4-inch pipe can convey the trash from a 3-inch pipe.

2. Can You Drain A Toilet Into A 2-inch Pipe?

Unless two toilets share a drain (you will need a 4-inch sewage waste pipe), the toilet demands a 3-inch drainpipe. Facilities with less than nine units could employ a 2-inch pipe unless there is a toilet discharge into the pipeline. 

3. How Deep Does A Toilet Drain Need To Be?

The minimal size for a toilet drain is 3 inches, irrespective of your plumbing regulations. 

4. Should There Be Standing Water In The Toilet Drain Pipe?

Your sewage drains should trap water in specific areas. The water in the plumbing system keeps vermin and odors out of the premises. 

A toilet pan, for instance, always has water in its basin to deter unpleasant odors from the building. 

5. How Much Slope Does A Toilet Drain Need?

Plumbing regulations require you to slope toilet drain pipes within a 1/2 to 3 inches per foot or vertical. An inclination of less than a 1/4 inch per foot will cause several drain blockages, but a slope of at least three inches would enable the water to drain. 

6. Why Is Toilet Paper Coming Out Of The Outside Drain?

If your outdoor sewer is flooding, you understand the clog is downstream of it. If the drain disconnects from the primary waste line from your apartment, you will notice a clog between the drain line and the sewer or septic tank. 

7. What Size Sewage Pipe Do I need?

The regular sewage drain pipe size in a home with one or two toilets is 3 inches. The sewage pipe attaches to the toilet flange underneath the toilet- and the flange measures 3 inches. 

8. How Deep Is A Toilet Drain?

The most popular toilet rough-in size is 12 inches. Nonetheless, it’s worth noting that 10-inch and 14-inch rough-ins are also available. 

Read also Best 10 inch to 12 inch toilet adapter

Closing Thoughts

From the post, plumbing regulations set 3 or 4 inches diameter as the correct size for toilet drain pipes. You will also need proper tools for the task and foreknowledge if you choose the DIY path. 

In case of any queries or clarification, you can revisit this post or contact professional plumbers near you.

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