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Toilet Trapway Size: What Is It and Why It Matters?

Incorrectly installing a toilet trapway can damage your plumbing. You will need to do more manual labor or call a plumber to fix it. So, it could mean an extra cost to you.

Toilet trapway size matters so your toilet can flush and function properly. A trapway that is too narrow or too wide can lead to clogging, weak flushing, and other impactful issues.

Let’s look at what a toilet trapway is, why the size matters, and other information worth knowing about your toilet.

Toilet Trapway Size

What Is a Toilet Trapway?

A toilet trapway connects your toilet to the sewer pipes. You may also know it as an outlet because it’s the small tube that begins at the hole of your toilet bowl. 

The trapway is where waste goes and leaves your home to get to the sewer when you flush. Sometimes they are not visible, so you may not have recognized yours. But trapways are crucial to the flushing process.

What the trapway looks like doesn’t matter. The size of the trapway plays a massive role in the functions of a toilet. It can be the difference between an overflowing toilet bowl and a toilet with zero issues.

Why Does Toilet Trapway Size Matter?

There are several functional reasons why toilet trapway size matters. 

Your toilet trapway size matters because if it is too narrow, your toilet will constantly clog. 

Keep in mind that biological waste isn’t the only contributing factor to a backed-up toilet. Cigarette butts, napkins, tampons, and other garbage can find their way into your toilet. When attempting to flush these objects through a narrow trapway, the small diameter of the trap may not be able to let the objects through. 

This would inevitably lead to a nasty clog that can cost a lot of money to remedy. 

Another reason is a concept called siphoning. A toilet trap should be a particular diameter so that the water flow from the toilet tank will be enough to trigger the siphoning effect. Too big of a trapway would need a wasteful amount of water for a complete flush. 

To summarize, trapway size influences:

  • Flushing rate
  • Reliability
  • Size of objects that can be flushed

Does the Flushing System Impact Trapway Size?

Yes. The type of flushing system, siphonic or washdown, determines trapway size you need. 

Siphonic Flushing

Siphonic flushing is the most popular type of toilet flushing. This process exploits gravity to suck out the water from the bowl continuously until all the water from the tank runs out. 

Eventually, air will enter the system and stop the siphoning process. This is why you hear a gurgling sound when you successfully flush the toilet.

If your trapway is too large, it can affect the siphonic flush and limit its effectiveness.

Washdown Flushing

Washdown flushing also uses gravity, like siphonic flushing. But instead of a vacuum, this method uses weight. It sends wastewater right down as opposed to a curved trapway.  This form of flushing utilizes a wider trapway because of the higher water flow used in the design. A narrow trapway will only lead to clogging and won’t work with the toilet hole size that’s optimal for this method.

You’ll typically find these types of flushing in commercial establishments and public spaces. They have a low probability of clogging thanks to the large toilet drain.

Washdown Flushing system

What Are the Different Sizes?

With this information in mind, let us look at all the different trapway sizes. This is where it can become a tricky process when repairing a trapway or choosing one for your toilet. Toilet trapways come built into the toilet you buy.

Trapway sizes are very close in size, only ¼ of an inch separating some. With that said, the standard trapway size for home toilets is 2-inches in diameter. Another common trapway diameter is 2⅜ inches. It’s only a bit wider than the standard, but that difference can go a long way in terms of reliability. 

This one is the leading size because it makes for a clog-free system. It is still small enough to use siphonic flushing well and fits solid biological waste of most sizes. 

The widest available trapway size is a 4-inch diameter, a full 2-inches more than the standard. This diameter is not ideal for siphonic flushing but is perfect for the washdown method.

Of course, there are other sizes than these main three, like 3-inch diameters and 2⅛-inch diameters. Once you consider what flushing system and general trapway size you need, you can determine what toilet would work best for you. 

Toilet Trapway Visual Designs

When it comes to visual design, there isn’t a lot to choose from. You can choose between concealed trapways and exposed trapways.

Concealed Trapways

You’ll find these common designs in your home improvement store. The trapway system sits inside the toilet’s housing, away from view. The advantage of this design is that the sides are easier to clean due to the smoother sides which conceal the system. 

Exposed Trapways

If you’re looking for a toilet that adds some pizzaz, consider one with an exposed trapway. Instead of hiding the workings of the toilet, these incorporate the trapway into the aesthetic to add visual appeal. The downside is that they are harder to clean because of the curvatures added to the toilet’s body.

What Are the Different Trapway Types?

There are two trapways, an S trap, and a P trap. They both function to create a barrier between your toilet and the sewer. The real difference between them is their shape and effectiveness.

S Trapway

The S-trap looks exactly as the name describes. The water flows along a path that takes the shape of an S. The main issue with this trapway is how it can allow too much water past it and into the sewer. This issue could leave a gap at the mid-line of the trapway. 

An air gap creates space for gases to pass through, which can not only lead to odor issues but sanitary ones. These gases can be dangerous and make you and the people in your home sick. 

P Trapway

The P trap removes the issues that the S-trap has. By adding more piping, it fixes the issue of excessive water flow. It’s an efficient flushing process that consistently gives way to more water. The water acts as a barrier against gasses. Read more about a P trapway here.

How To Find Toilet Trapways

Toilet trapways tend to come attached to the toilet you purchased. Some toilet designs sell trapways separately in case you need a replacement.

Toilets that come with a specific trapway tend to be built for a certain flushing method. But you can get the individual trapways at most home improvement stores. Or you can buy them from the toilet manufacturers themselves (if you’re having trouble with your flush, see our article ‘Toilet Flushes But Waste Comes Back’).

An example is this toilet trapway from Kohler, that you can buy separately.

How to Unclog a Toilet Trapway

How to Unclog a Toilet Trapway

Now you know what toilet trapways are and why their sizes matter. Now, it’s worth learning how to solve a blockage problem caused by improperly flushing the toilet. For instance, you could have expected your home toilet to flush as efficiently as public toilets that use the washdown method. 

You don’t need expensive tools to unclog an S-Trap toilet. Grab a plunger and submerge it gently into the bowl. Ensure that the lip of the plunger cup surrounds the base of the trapway. Slowly push the plunger into the trap without any sudden jerking motions. This is will avoid any nasty splashing

Hopefully, after a few seconds, you’ll see the water receding down the trap. Then, you can attempt to flush the toilet.

If your toilet trapway is clogged with something bigger, like a kid’s toy, a feminine product, or anything else accidentally flushed down the toilet, a plunger might not cut it. You may need to buy a toilet auger.

A toilet auger or toilet snake is designed with flexible metal. This material allows it to snake its way inside the trapway. Insert the toilet snake’s shaft into the trapway. Once it’s all down there, hold the handle with your other hand and turn it continuosly. 

You could also push and pull on the handle repeatedly to push down what’s causing the blockage. Finally, you can combine the two motions if you want to ensure that the entire blockage is released.

It’s worth noting that both these methods of unclogging a toilet trapway work for toilets with both S-Traps and P-Traps.


A toilet trapway is an integral part of your toilet. It efficiently flushes down any biological solid waste through the elegant use of gravity. 

But it isn’t the first thing that comes to mind when homeowners buy a replacement toilet. That’s why it’s essential to know the basic workings of your toilet trapway so you know how to fix any issues that come up when a toilet is misused.

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