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How Long Can a Sink Drain Pipe Be? Why Does it Matter?

Drain pipes are found in most plumbing fixtures and are used for a water passage to the sanitary pipes. It includes various parts, including a P-trap, tailpiece, and drain stub out. But for the drain pipe to work, it has to be of the correct length, especially the drain tailpiece.

So, how long can a sink drain pipe be? According to the trap requirement under code 1002.1, the vertical length from the outlet to the trap should not exceed 24 inches. The horizontal distance should not be more than 30 inches.

The article covers more information on a sink drain pipe length, helping to explain how long it should be. Read on to learn more and understand why drain pipe length matters.

How Long Can a Sink Drain Pipe Be?

Sometimes, the term drain pipe is used to represent all the parts in the pipework under the fixture that connects to the sanitary pipe. These parts include the tailpiece, the pipe that connects the fixture outlet, and the other parts, the P-trap and the drain stub out.

However, the term mainly means the total length of the drain tailpiece. In this case, the sink drain pipe size should not exceed 24 inches. This standard is per the International Residential Code, under trap requirement code 1002.1, on fixture traps. In Canada, the distances should be a maximum of 36 inches.

how long can a sink drain pipe be

Likewise, the horizontal distance measured from the fixture outlet to the trap inlet should not exceed 30 inches.

The measurement for the horizontal distance is from the centerline of the fixture outlet and the centerline of the p-trap entrance. The trap prevents the gases from the sewer from entering the house.  

Why Does the Length Matter in a Sink Drain Pipe?

The length of the sink drain pipe matters and is crucial to the drainage system.

  • One of the reasons is to maintain an adequate waste velocity when water runs through the trap.
  • If the velocity is low, the p-trap may experience self-siphonage, a condition caused by the pull exerted by the moving water column. When this happens, gases from the pipe system may rise through the drain pipe into the house with unpleasant odor from the pipe system.
  • Also, if the drain pipe is too short, the back pressure from the drainage system could cause a blowout in the trap seal.
  • When the water runs down through the pipes, it comes down with pressure, some of which flow ahead of it. If the pressure finds a blockage in the system, this pressure stops and pushes back through the pipe.
  • Thus, the back pressure can cause a displacement in the trap way and even cause water to flash back up.
  • Another reason the drain pipe should be of ideal length is to prevent a hydraulic jump. The jump happens when the flow rate exceeds the drain capacity. 

The instance causes the flow depth to be more than the pipe diameter. A hydraulic jump can block the airflow in the drain pipe and with time, generate back pressure.

Can One Use a Sink with a Long Drain Pipe?

Yes. You can use a sink with a long drain pipe and even a drain pipe with a p-trap under the floor. This instance is possible, like when using an island sink, usually placed in the middle of the kitchen.

  • However, the vent should be nearby for the sink to work, probably at the kitchen wall. The vent allows fresh air to flow through the drainage pipe, allowing the water to flow downwards freely.
  • Nevertheless, it must be permitted by local codes for a plumber to use a sink with a long drain pip. For instance, you can use up to 36 inches of long tail pieces in the drain pipe in Canada.
  • The long tailpiece length allows the vent pressure to push down the water in the system. The vent usually connects to the drain pipe after the p-trap.
  • You can stick to the short tailpiece if the long drain pipe is not allowable in your state. The case can also be that you don’t have a space below the floor framing to access the p-trap.
  • In this case, the vent should be in two positions in the drain pipe. One near the end of the drain pipe, usually under the floor framing. The other connection is shortly after the p-trap, where the water flows to the sanitary pipes.
  • The vent height must be higher than the outlet from the plumbing fixture. In the case of an island kitchen, the loop vent should run up at the side of the sink to allow more pressure to build.

 How Long Can a Drain Pipe Trap Arm Be?

As mentioned earlier, some plumbers may refer to the trap arm when discussing drain pipe length. The trap arm, mentioned as stub out earlier, is the part after the p-trap that connects to the sanitary pipes, usually with the vent.

The diameter of the pipe used usually determines the length of the trap arm. So, the trap arm should be at least two times the diameter. The following table shows some of the trap arm diameters and the maximum length they should have.

The info in the table below is by one jurisdiction’s rule, which varies with the state.

Trap Arm DiameterMaximum Trap Arm Length
1.25 inches5 feet
1.5 inches6 feet
2 inches8 feet
3 inches12 feet


Following are the frequently asked questions on drain pipe length and how long it can be.

Q1. What Happens When a Drain Pipe Trap Arm Is Too Long or Short?

If the trap arm is too long from the trap, it can cause it to siphon. If the trap arm is too short, the water flowing after the p-trap may run up the vent clogging it.

Q2. How Long Can a Horizontal Sink Drain Pipe Be?

For a 1.25-inch pipe, the horizontal distance of the pipe should not be longer than 30 inches (5 feet) to the vent. A 1.5-inch pipe should have a maximum length of 42 inches (3.5 feet). However, this may vary with state, 

Q3. How Far Should a P-trap Be Below The Sink?

In most states, the P-trap should be at least 24 inches from the waste’s exit point under the sink. This length is usually the tailpiece length. However, in some countries, the length can be up to 36 inches.

Final Thoughts

For a plumbing fixture to work ideally, it must have the correct lengths between its various points. The length is crucial to allow the pressure from the vent to work in the system forcing waste down. In most states in the US, the length should be 24 inches.

However, when you may be referring to the trap arm length, it should be at least two times the pipe diameter. Failure to use the correct drain pipe lengths can result in many drainage issues, including blockages and, worst case, a blowout.