What Size Drain Pipe Is Used For A Kitchen Sink And Dishwasher?

Dishwashers are great home appliances that simply make life easier. If this is your first dishwasher or your first time replacing one, you might wonder what size drain pipe is needed for a kitchen sink with an attached dishwasher.

The plumbing code in most states requires a combined discharge from a kitchen sink and dishwasher to be at least 1 ½ inches in diameter. However, you may have to use a larger pipe size, especially if your kitchen sink is older. A larger pipe may also be needed if the drain must serve the sink and two or more appliances.

What Size Drain Pipe Is Used For A Kitchen Sink And Dishwasher?

A kitchen sink and dishwasher drain pipe should be at least 1 ½ inches in diameter. However, the minimum size can vary from state to state. Check your local codes before installing or altering your plumbing system. 

The Uniform Plumbing Code (UPC) is developed, maintained, and published by the International Association of Plumbing and Mechanical Officials (IAPMO). It has been adopted as law, in whole or in part, by almost every state in the U.S. 

Thirty-five states have taken the further step of adapting the International Plumbing Code (IPC), which is slightly more in-depth than the UPC.

These codes are established by experts taking into consideration the latest research and technology developments, and even allow room for emerging technologies. The codes they propose are designed for the safety and benefit of the community as a whole as well as the environment. 

How to Measure Kitchen Sink Drain Size

Modern kitchen sinks have a drain that is 3 ½ inches in diameter. However, there was no real standard size with older models. This means if you are doing a period or historic remodel or are working with the original sink in an older home, the drain size may differ. 

If you are dealing with a larger drain or a different sized drain on an older model of sink, keep in mind the drain pipe is sized in proportion to the sink drain. You therefore may need a different size than the normal kitchen sink drain of 1 ½ or 2 inches. A plumbing drain pipe size chart can help you figure out which drain size you need.

New residential sink drain pipe size in no case shall be less than 1 ¼ inches, regardless of the sink model or placement. 

To measure your kitchen sink drain, use a tape measure and record the length from the outside of the ring to the outside of the ring on the opposite side. Then record the depth of the drain, measured from the surface level of the sink to the deepest part of the drain opening. This will give you an accurate measure for your drain as well as the dimensions for a crumb cup.

How to Connect a Kitchen Sink Drain Pipe

It is very important before you begin to make sure that all of the parts you are using are compatible with each other and your existing equipment. 

Measure all of the needed pipes and fittings. Most people opt to use copper or PVC pipe for kitchen sinks, so you will want to use a PEX or PVC cutter to size the pipe appropriately. If you aren’t comfortable using those tools, try using a hacksaw instead. 

Be sure that everything fits and matches up correctly if you had to cut any holes to fit faucets or auxiliary pieces. Reference a kitchen sink drain pipe diagram if you need to in order to be sure of how everything will go together. 

Clean the existing wall pipe that you will be attaching to with a rag, if necessary. You can whisk away mild corrosion with a wire brush. However, if you find damage from corrosion you may want to replace the drain pipe in the wall before connecting your kitchen sink drain pipe. 

1. Assemble the Drain

Using a stain-free plumber’s putty, seal the drain securely to the sink. Remove any excess putty before it dries. 

Tighten the large nuts under the sink to secure the drain. You can buy a special wrench designed for this, but it is not necessary in most cases. 

2. Assemble Pipes and Fittings

Begin assembling the pipes, but wait to tighten them until you are sure that everything is in place. 

If you have a double sink, you may need to use a T-shaped fitting to configure your drain pipe. Your drain pipe also must include a P-trap to keep gasses from seeping into your home. 

There may be a pipe from the P-trap to the wall drain pipe or you may need to connect it directly depending on the exact configuration and the space available. 

3. Tighten Fittings

Tighten all of the fittings taking care that everything is sealed completely. Use Teflon tape over the threaded connections at the end of each piece and with fittings to ensure a water-tight seal.

4. Test the Connections

Go ahead and run a small amount of water through the sink drain. Watch closely underneath for any leaks.

It is a good idea to use a towel or a few paper towels to cover the area in case a leak does occur. You will want to address any leaks immediately before you consider the job finished. 

How to Plumb a Single Kitchen Sink With Disposal and Dishwasher

It is important to note that you will need a different strainer for a sink with garbage disposal than for one without. You will want to begin as above by installing the drain strainer and cleaning the existing wall drain pipe if it is used. Once this is done you can begin installing your appliances.

1. Install Garbage Disposal

The garbage disposal should slide right into place. It is held in place with a gasket and a clip. 

2. Assemble Pipes and Fittings

Just like above, assemble all of your pipes and fittings, making all necessary adjustments. You will need to include a P-trap between the garbage disposal and the drain pipe in the wall, just as above. 

3. Tighten the Fittings

Make sure all of your connections are tightened, using Teflon tape as necessary. 

4. Connect the Dishwasher

Your dishwasher will connect directly to your garbage disposal. It must run through the garbage disposal after it runs through the air gap fitting for everything to work together correctly. 

There will be directions with your disposal from the manufacturer that will tell you specifically how to make this connection. Refer to a kitchen sink drain pipe diagram if you need to. 

5. Test the Connections

Again, you will want to do a small trial run to make sure that everything is connected and that there are no leaks.

Once you are sure there are no leaks, test your garbage disposal. When you are sure the disposal is running correctly, run your dishwasher to see that it runs correctly as well. 


Can I Use Schedule 40 PVC for Sink Drain?

Yes, you can use schedule 40 PVC for sink drains. Many people find PVC to be preferable for sink drains because PVC is very easy to use, does not corrode, and in rented homes has no value for thieves to steal. 

What Is the Size of P-Trap Required for a Kitchen Sink?

P-traps come in the same dimensions as the pipes you plumb your kitchen sink with. You will want to make sure that you use the same size P-trap as you do pipes in order for everything to fit. For example, if you have installed a drain line that is 1 ½ inches then you will want to use a P-trap that is 1 ½ inches in diameter. If you have a two-inch drain line, then you will need to use a P-trap that is two inches in diameter and so on. 

How Many Fixtures Can Be on a 3 Inch Drain?

A 3-inch drain allows for 48 drainage fixture units (dfus). Drainage fixture units are a measurement of how much load pipes can handle. As long as your fixtures combined do not put out more than 48 dfus, you may use a 3-inch drain. 

Can a Washer and Kitchen Sink Share a Drain?

A washer and a kitchen sink can share a drain as long as they are connected to the vent properly. In some cases, this may mean each appliance has its own vent. 

To End

Selecting the right drain size for a kitchen sink and dishwasher allows you to prevent plumbing and drainage problems. The actual pipe size can vary based on the type of sink and the number of appliances you want to hook to the same drain. However, the pipe can’t be smaller than 1 ½ inches. We hope this guide can help you select the right pipe size for your setup. 

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