The basic setup: what changes when a dishwasher is in the picture

Installing a garbage disposal with a dishwasher present means running a drain line from the dishwasher into the disposal unit instead of directly into the sink drain. The disposal itself installs the same way — you mount it under the sink, connect it to the sink drain, and wire it to power — but you add one extra step: connecting the dishwasher's drain hose to a port on the side of the disposal.

Most disposals sold today have a dishwasher inlet port already built in, usually a small threaded opening on the side. If your disposal does not have one, you will need to drill and tap the opening yourself, or buy a disposal model that includes it. The dishwasher connection is straightforward once the disposal is mounted; it takes about 15 minutes and requires only a hose clamp and the drain hose that came with your dishwasher.

Key Takeaways

  • Buy a garbage disposal with a dishwasher inlet port already drilled, or plan to drill and tap one yourself if your model lacks it.
  • Install the disposal under the sink first, then connect the dishwasher drain hose to the inlet port using the hose clamp that comes with the disposal.
  • You will need a screwdriver, adjustable wrench, hose clamp, plumber's putty, and a drill with a tap set if the inlet port is not pre-drilled.
  • The dishwasher hose connects to the disposal, not directly to the P-trap, so water from both the sink and dishwasher flows through the same disposal unit.
  • Test the dishwasher and disposal together before you finish; water should drain without backing up into the sink.

What you need before you start

Gather these tools and materials: a screwdriver (Phillips and flathead), an adjustable wrench, plumber's putty, a hose clamp (usually 5/8 inch), a bucket to catch water, and a flashlight. If the disposal model you buy does not have a pre-drilled dishwasher inlet, you will also need a drill, a 7/8-inch hole saw or spade bit, and a 7/8-inch NPT tap with a tap wrench.

Before you buy the disposal, measure the space under your sink. The unit needs clearance on all sides — at least 6 inches below the sink drain opening and enough room to reach the mounting bolts with a wrench. Check whether your sink has a single or double basin; if it is a double basin, the disposal typically goes under one side, and the other drains directly into the P-trap.

Turn off power to the outlet under the sink, or plan to do so before you plug in the disposal. If there is no outlet there, you will need an electrician to install one, or you can use a plug-in cord with a GFCI outlet. Have the dishwasher manual nearby so you know the diameter of its drain hose — most are 5/8 inch or 3/4 inch.

Removing the old drain and preparing the sink opening

If you are replacing an existing drain, unscrew the sink strainer from above and below. Place a bucket underneath, then unscrew the tailpiece (the pipe that hangs down from the sink) and the slip nut that holds it to the P-trap. Pull out the old strainer and clean the opening with a rag.

Scrape away old putty from the sink opening using a putty knife or old screwdriver. Wipe the rim clean with a damp cloth and let it dry. This step matters because new putty will not seal properly over old debris, and water will leak into the cabinet.

If you are installing a disposal for the first time and there is currently no drain under the sink, you will still need to remove the sink strainer and install a new one that is compatible with the disposal. Most disposals come with a mounting ring and a new strainer basket.

Installing the disposal unit itself

Roll a rope of plumber's putty about the thickness of a pencil and press it around the underside of the new sink strainer. Push the strainer up through the sink opening from below, then press it down firmly from above so the putty seals the gap. Wipe away excess putty with a wet rag.

Screw the mounting ring onto the strainer threads from below. This ring holds the disposal in place. Slide the disposal unit up into the mounting ring and rotate it clockwise until it locks. The disposal should hang securely and not move when you push it.

Connect the tailpiece (the short pipe from the strainer) to the top of the disposal using the slip nut and washer that come with the unit. Hand-tighten first, then use an adjustable wrench to snug it — do not over-tighten, or you will crack the plastic. Connect the bottom outlet of the disposal to the P-trap using a new slip nut and washer. Again, hand-tighten and then wrench-tighten gently.

Adding the dishwasher drain connection

Locate the dishwasher inlet port on the side of the disposal. If it is already drilled and threaded, skip to the next paragraph. If it is not, use a 7/8-inch hole saw or spade bit to drill through the marked spot on the side of the unit. Tap the hole with a 7/8-inch NPT tap, turning the tap wrench clockwise until the threads are cut. Back the tap out by turning counter-clockwise, then remove it.

Screw the inlet fitting (usually included with the disposal) into the threaded port. Hand-tighten it, then use a wrench to snug it. Do not over-tighten; the fitting is plastic and will crack.

Slide the dishwasher drain hose onto the inlet fitting. The hose should fit snugly. find it with a hose clamp — loosen the clamp screw, slide the clamp over the hose and fitting, then tighten the screw with a screwdriver until the clamp is snug but not crushing the hose. The hose should not slide off when you pull gently.

Wiring and testing

Plug the disposal into the outlet under the sink, or have an electrician wire it if there is no outlet. The power cord usually has a three-prong plug. Do not hardwire the disposal to the circuit; a plug allows you to disconnect it safely for repairs.

Before you run water, check all connections by hand. The tailpiece should not move, the P-trap connection should be tight, and the dishwasher hose should be clamped securely. Place a bucket under the P-trap connection to catch any drips.

Turn on the cold water at the sink and let it run for 10 seconds. Watch underneath for leaks. If water drips from the tailpiece or P-trap connection, tighten the slip nut a quarter turn with a wrench. If water leaks from around the strainer, you may need to add more putty or re-seat the strainer.

Once the sink drains without leaking, run the dishwasher through a short cycle. Listen for the drain pump to set up, and watch the sink to make sure water does not back up. If water pools in the sink while the dishwasher drains, the disposal inlet may be blocked, or the hose may be kinked. Check the hose path and make sure it is not bent sharply.

Common problems and fixes

If water backs up into the sink when the dishwasher runs, the disposal may not be grinding properly, or the inlet port may be blocked. Turn off the dishwasher and disposal. Remove the hose from the inlet port and look inside with a flashlight. If you see food or debris, use a straightened wire coat hanger to fish it out. Reattach the hose and test again.

If the disposal leaks from the bottom, the seal inside the unit has failed and the disposal will need to be replaced. This is not a repair you can do yourself. Drain the water from the P-trap into a bucket, disconnect the disposal, and install a new one using the same steps.

If the dishwasher hose leaks where it connects to the inlet port, the hose clamp may be loose or the hose may be cracked. Tighten the clamp first. If water still leaks, slide the hose off and inspect it for splits or punctures. If the hose is damaged, it will need to be replaced — contact the dishwasher manufacturer for a replacement hose.

Frequently Asked Questions

Can I connect the dishwasher drain directly to the P-trap instead of the disposal?

Yes, but it is not recommended. Connecting directly to the P-trap means food particles from the sink will accumulate in the P-trap instead of being ground up, leading to clogs. The disposal inlet is designed to handle this; the P-trap is not.

What if my disposal does not have a dishwasher inlet port?

You can drill and tap one yourself if you have a drill and a 7/8-inch NPT tap set, which costs about $15 to $30. If you are not comfortable drilling into the disposal, buy a model that comes with the port pre-drilled. Most modern disposals include it.

Do I need a check valve on the dishwasher hose?

Some dishwashers come with a built-in check valve in the hose to prevent backflow. If yours does not, you can buy a separate check valve and install it on the hose near the disposal inlet. This prevents dirty water from the disposal from flowing back into the dishwasher.

How tight should the hose clamp be?

Tight enough that the hose does not slide off when you pull it gently, but not so tight that you crush the hose. If you can turn the clamp screw with your fingers, it is too loose. If you need a wrench to turn it, it is too tight. A screwdriver should require moderate pressure.

What if water leaks from under the sink after installation?

Check three places: the strainer seal (tighten or re-putty), the tailpiece connection (tighten the slip nut), and the P-trap connection (tighten the slip nut). Tighten each by a quarter turn and test again. If leaking continues, the seal inside the disposal may have failed and the unit will need replacement.