can a toilet and sink share the same drain
Kristina Davis

So you have a sink drain and toilet drain, but are you asking ‘can a toilet and sink share the same drain?’

It’s an interesting question, and the answer depends on your plumbing code. The great thing about plumbing is that it’s an inherently modular system. 

So if you live in an apartment or condo where there are no separate bathrooms and kitchens. Then it might be a good idea to connect your sinks and toilets.

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The short answer is yes, a toilet can share a drain with a sink. But there are generally some caveats and considerations to keep in mind, which we’ll get into in this article.

Why You Might Want To Connect The Toilet To The Sink Drain

why connect toilet to sink drain

If you have a sink in your bathroom, it’s likely that the drain is connected to the toilet. This is not necessarily a bad thing! 

You may want to use the sink for washing your hands before going out into society and eating food (or drinking soda). Or after coming back from dinner with friends. 

Similarly, if you have a large enough basin for washing face and body parts at once.

Or, because of an oversized bathtub, it can be helpful to connect your commode directly to this pipe. 

This is so you don’t have to stop midway through cleaning yourself up when someone else needs access to their restroom facilities as well.

Tip: It's also important to note that you can connect a toilet to a sink drain line, but you'll need to follow local regulations and manufacturer's instructions.

So, Can A Toilet And Sink Share The Same Drain?

tooilet sink share same drain

The first thing you need to know is that both toilets and sinks are connected to the same drain line.

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This means that the water flow from your toilet will enter the sink’s drain through your plumbing system. 

Then continue on its way until it reaches an open manhole or overground trench. When it reaches there it empties into sewage treatment systems or waterways.

If you have multiple toilets in your house or apartment building. Then each one could potentially be connected via this shared line at some point during its journey through pipes. 

After which they are discharged into rivers or oceans outside your property’s property perimeter limits.

Considerations To Note Before Connecting Toilet And Sink Drainage

You can connect or add a toilet using the same drain line as your sink or vanity.

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There are some considerations and regulations that you have to follow, but the process is reasonably simple.

considerations for drain

The first step is to decide whether you want to install a new toilet in an existing space (like an alcove) or if it’s preferable to relocate it to a new location.

If you’re going for efficiency and space savings, then it might be best for all parties involved!

Ensure that there’s a vent line between these two valves on each side of your house. This vent line will allow airflow through it. 

Tip: The vent line will help keep things clean and dry if you have any damp issues with either system (i.e., mold or mildew).

How To Connect A Toilet And Sink Drain

connect toilet and sink

Toilet and sink drains can be connected by using a reducer.

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This is a small piece of plumbing that allows you to connect different-sized pipes together. And it will allow the two drains to share water flow.

Toilet-to-sink drain connections are fairly simple and do not require a plumber or professional to do so for you.

Here is how:

Step #1: Sink Trap

The sink trap is a U-shaped bend in the drain pipe that allows water to flow into the sink but prevents sewer gases from escaping. And it consists of two parts. 

The trap seal, has a larger diameter than most other sections of pipe. It is usually about 1 1/2 inches or greater.

Then inclusive is a smaller section called your “sink strainer,”.

This is basically just another part of your drain that sits at right angles to your toilet’s waste lines.


Step #2: Venting

The bathtub drain is ventilated to prevent sewer gases from coming up through the drain.

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If you don’t vent it properly, there will be problems with your plumbing and you could have a nasty smell in your bathroom.

Venting also helps prevent sewer gases from coming up through the toilet and sink drains if they share a common venting system.

This is common, especially for those in bathrooms.

Your home has pipes that may have different diameters (and thus different flow rates).

It then may be necessary to install another size of vent pipe before connecting the two fixtures. 

Tip: This will make sure there's enough room in both pipes for water pressure and flow rate requirements.

Do Toilets And Sinks Use The Same Drain?

toilet and sink use same drain

Do you ever wonder why the pipes at your house are so long? 

Well, the answer is simple: because they drain into the sewer system or the septic tank if you have one. 

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But what about sinks and toilets? You might be surprised to learn that these two fixtures also use drains connected to sewers or septic tanks.

Yes, they both use the same drain.

The sink and toilet are connected by a pipe that runs out of your home and into the sewer system. This is called a “drain” or “plumbing line.”

plumbing in bathroom

The plumbing between your home and the city’s wastewater treatment plant may be underground.

But it’s still connected to your home in some way. You just need more pipes than most people think. 

Toilet and sink drains are different, so the water that flows from your toilet into the sink is not going to be the same as what comes out of your drain.

You have to ensure that the installation process doesn’t damage either piece of your plumbing equipment! 

Tip: If those things can be handled by an experienced plumber then do it!

Can You Tie A Sink Drain Into A Toilet Drain?

tie sink drain into toilet

You can tie a sink drain into a toilet drain if you want to.

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The first thing you should know is that the size of your sink drain and the size of your toilet drain is not the same. 

Sink drains are larger than toilet drains, so they can handle more water before they overflow.

Another key difference between a sink and a toilet is that sinks don’t have enough space for wastewater to flow out through spigots as toilets do. 

They just dump their contents directly into sewer lines via gravity alone!

Drain Into A Toilet Drain, & Check Plumbing Codes

Make sure you check the plumbing codes with your local plumbing inspector.

They will be able to tell you if it is allowed. And if there are any other restrictions or safety concerns.

If it is allowed and the codes are followed properly, then no problem! 

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check codes for plumbing

This is because some codes don’t allow the two drains to be connected.

While some jurisdictions require that the water from the sink drains into a separate septic tank.

The septic tank is not connected to the toilet drain, so it must be pumped out every few years.

Lastly, codes will also tell you how far apart the toilet and sink must be for the two drains to be connected.

Tip: You need to make sure that your plumbing is up to code and has been installed correctly, as well as follow all local codes and regulations.


So, can a toilet and sink share the same drain? The answer is yes, but you may want to think twice about it.

There are some jurisdictions that do not allow this. And others that require separate disposal systems for wastewater from sinks and toilets.