how many toilets on a 3 inch drain

How Many Fixtures Can Be On A 3 Inch Drain

Being a homeowner comes with huge responsibilities as there are many things you must take care of to ensure your home runs smoothly. Different aspects like knowing the number of toilets that a 3-inch drain handle can help make your work easier.

How many bathroom can be on a 3 inch drain?  A 3 inch drain is perfect for 3 toilets because you can have a maximum of 20 Drain fixture units on a 3 inch drain. Drain fixtures is a unit of reference to units that you can connect to the drain line and a toilet represents about 3 drain fixture units.

However, because you are not only connecting a toilet but a bathroom sink, bathtub and others in your home you cannot say 5 rather you must include the other DFUs. Below we have discussed this further.

How Many Drain Fixtures in a 3 inch Drain?

A 3 inch drain can have a maximum of 20 drain fixtures as recommended by the drainage system sizing code. 

The math behind is that for a 3 inch drain you are allowed a maximum of 20 drain fixture units. Which include your kitchen, bathroom sinks, bathtub, drain. Often a toilet represents about 3 drain fixture units, however the whole 3 inch drain carries waste from the sink, bathtub ,washer etc. 

Which means for a toilet , bathroom sink and bathtub the total can be 5-6 drain fixture units. Hence you can have 3 toilets for a 3 inch drain in your house.

Can Two Toilets Share The Same Drain?

how many toilets on a 3 inch drain

You can use a single drain for two toilets provided they are located on the same part of your home but remember they cannot share one drain hole. However, if not, you will need to use different drains. 

To attach two toilets to one drain, the plumber will have to put the soil stack running to the horizontal line of drainage vertical. Moreover, the downward slope of the connecting pipe between the main sewer line and the stack should be a quarter-inch per foot. 

A clean-out must also be fitted where the vertical stack becomes horizontal. In addition, a closet flange will be used to join the drainpipe and the toilet drain. 

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Can A Shower And Toilet Use One Drain?

You can use one drain for your toilet and shower, but these facilities cannot use the same waste trap arm because of hygiene and sanitary reasons. Wastewater from toilets and showers is channelled to treatment facilities, although water from the shower would drain outdoors. 

However, showers and toilets generally use different drains because of the wastewater each produces (Blackwater and greywater). 

Greywater is wastewater produced in office buildings or homes free of faecal contamination. It has fewer pathogens and is generally easier to reuse and treat.

However, greywater can still have pathogens from cleaning yourself while showering. Never dispose this way into a drinking source, and that includes lakes, streams, or rivers.  

Some people reuse this water to wash their cars or water plants but be careful about how much chemicals are exposed to as it could kill them.

On the other hand, blackwater contains urine and faeces, making it a severe sickness and disease vector. Stringent measures govern the disposal of this wastewater.

You cannot dispose of blackwater yourself since it must go to a sewage treatment plant. You could incur colossal fines or even get arrested if you dump it in a stream or lake. 

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What Toilet Drain Size Should You Use?

While the standard size of drain pipes is 3 inches, the popularity of 4-inch pipes is on the rise. The one-inch difference may not sound like a big deal for many people, but it makes a difference. 

A 4-inch pipe moves more wastes compared to a 3-inch drain pipe; that’s why 4-inch pipes are usually used for multiple toilet waste removal. However, a 3-inch drain is enough for your home unless you generate a lot of waste.

A professional plumber is better equipped to tell you the size of drain suited for your toilet. 

Can You Put a Toilet On a 3-Inch Drain Pipe?

This is the standard size, and it is popular because it is efficient and cost-effective. 3-inch pipes are common in older houses, and they can be combined with 4-inch main drain pipes. 

It is also cheaper to install this type of drain pipe, and they are less cumbersome. However, 3-inch drain pipes are more susceptible to clogging than their 4-inch counterparts, but that shouldn’t be an issue with proper maintenance. 

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What is the Capacity of a 4-inch Drain Pipe?

A 4-inch drain pipe is superior to a 3-inch because it transports more waste and is less prone to clogging, but don’t use it as a trash receptacle. However, don’t flush anything else aside from toilet paper and waste, even on a 4-inch drain. 

You will find 4-inch drain pipes in settings where many toilets use one drain pipe; that’s why it’s the standard in most commercial buildings. 

However, these drain pipes consume more space and cost more to install. It is better to go with 3-inch drain pipes for small homes because putting 4-inch drains needs a bigger plumbing system.

Regardless of the size you choose, you can easily access any of these pipes but accessing a 4-inch drain pipe is easier since there is more space.

See also How to fix a toilet flange 1/2 above floor

FAQ on How many toilets on a 3 inch drain

1. Can You Put A Toilet On A 3 Inch Pipe?

You can use a 3-inch drainpipe on your toilet. In fact, it is common in older homes and is the standard drain line, although the 4-inch size is becoming used widely.

You can join a 4-inch main drain pipe and the toilet using a 3-inch drainpipe. 

2. Can 2 Toilets Share A Drain?

Two toilets can use the same drain pipe, but they cannot use one drain hole. 

3. How Many Toilets Can You Have On A 3 Inch Line?

According to the International Plumbing Code, a 3-inch horizontal branch can be connected to four bathroom groups or 20 DFUs. On the other hand, a vertical stack is good for multiple branches, with a single branch being restricted to 20 DFUs.

Read also Is drano safe for shower drains


You can have 3 toilet in a 3 inch drain pipe because the maximum number of drain fixture units on a 3 inch drain is 20 and a toilet represents about 3 dfu’s. Further because you will also have a bathroom sink and a bathtub too that means a bathroom needs 6 drain fixture units, meaning you can have 3 toilets on that drain.

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