P-Trap: What It Is, the Sizes You Need, and How It Works

A P-trap is the U-shaped bend of pipe under every sink, tub, and shower drain, and its one job is to hold a small pool of water that blocks sewer gas from rising into your home. That water plug is called the trap seal. Plumbing codes require a P-trap on nearly every fixture, size it to the drain, and ban the older S-trap because it siphons itself dry.

What is a P-trap and how does it work?

A P-trap works by trapping water in its lowest bend. Waste water flows in from the fixture, over the dip, and out to the drain, but a couple of inches of water always stays behind in the U. That standing water seals the pipe. Sewer gas coming up the drain line hits the water and stops instead of entering the room.

Turned on its side, the assembly looks like the letter P: the U-bend plus a short horizontal outlet called the trap arm. That trap arm is what separates a legal P-trap from a banned S-trap, and it is where the drain ties into a vent. The trap and vent work as a pair.

The trap also catches heavy dropped objects. A ring or a coin sinks into the U-bend rather than washing to the sewer, so shutting the water off fast often lets you recover it by opening the trap. For the whole under-sink picture, see our full kitchen sink drain assembly breakdown.

Why does a P-trap hold water?

A P-trap holds water because the U-bend sits lower than both its inlet and its outlet, so gravity keeps a pool trapped in the curve after every use. That pool is the trap seal, and its depth is regulated. Both the IPC and UPC require a seal of not less than 2 inches and not more than 4 inches, measured from the dip to the weir.

The 2-inch floor matters because a shallower seal siphons out too easily when a fixture drains. The 4-inch ceiling matters because a deeper leg traps solids and fouls instead of scouring clean each time water passes. Standard PVC and metal traps are built inside that window, so a correctly installed trap meets the seal depth without adjustment.

The seal only survives if air can reach the drain line behind it. That is the vent’s job, and it is why the trap arm ties to a vent within a set distance. Lose the vent path and a draining fixture pulls the seal down the pipe.

What size P-trap do you need?

P-trap size follows the fixture drain outlet. A bathroom lavatory uses a 1-1/4 inch trap, while kitchen sinks, laundry tubs, and bathtubs step up to 1-1/2 inch because they carry grease, food scraps, and larger surges. Showers usually run 2 inch. The trap arm that leaves the trap also has a maximum length to its vent, set by pipe size.

The distance runs from the trap weir to the vent connection, following the pipe centerline through any bends, and the arm must slope a uniform 1/4 inch per foot toward the drain. These figures track the common IPC and UPC tables, but exact limits vary by the code your state or local jurisdiction adopts, so confirm before you commit a rough-in. Our bathroom sink under-sink setup guide shows the lav side in detail.

P-trap vs S-trap: why are S-traps not to code?

A P-trap and an S-trap both hold a water seal, but only the P-trap vents. An S-trap drops straight down after the U-bend, so a basin of draining water pulls a vacuum behind it and self-siphons the seal out, leaving the trap empty and open to sewer gas. That single flaw is why both the IPC and UPC prohibit S-traps in new work.

The P-trap’s horizontal arm is what lets a vent tie in downstream of the seal. Air enters through the vent as water drains, which breaks the siphon and holds the seal in place. If you have an old S-trap under a sink, the common fix is to add a vent, often an air admittance valve where local code permits, and reconfigure the drain as a vented P-trap.

How do you install or replace a P-trap?

Replacing a P-trap takes about 20 minutes and a standard PVC trap kit runs around $8, which fixes the three usual under-sink problems: drips at the joints, a chronic slow drain, and a corroded metal trap. Match the new trap to the drain size first, then work slip nut by slip nut. Keep a bucket under the work.

1. Clear the cabinet and set a bucket or shallow pan under the trap to catch standing water.
2. Loosen the two slip nuts by hand, one at the sink tailpiece and one at the trap arm; use channel-lock pliers only if they are stuck.
3. Slide the old trap off and pour out the trapped water and debris.
4. Fit the new trap of the correct size, seating each beveled slip washer with its taper facing into the joint.
5. Set the trap arm so the U-bend hangs below the arm inlet and the arm slopes about 1/4 inch per foot toward the drain.
6. Hand-tighten each slip nut, then snug it a quarter turn with pliers; do not overtighten plastic threads.
7. Run water for a minute and feel every joint for drips, tightening slightly more only if one weeps.

If your drain layout does not match the parts, or the tailpiece and trap arm sit at odd heights, a flexible extension or a longer tailpiece usually solves it. See how the pieces relate in our guide to how drains, vents, and traps fit together.

Why does a P-trap dry out and cause a smell?

A P-trap dries out when a fixture goes unused long enough for the seal water to evaporate, often several weeks to a few months for a guest bath or floor drain. Once the seal is gone, sewer gas flows straight up the open drain, which is the most common source of a sudden sewer odor near a rarely used sink, tub, or basement drain.

The fix is fast. Run the tap for one to two minutes, or pour roughly a half gallon of water down the drain, to refill the U-bend and restore the seal. The smell usually clears within an hour. For a drain you almost never use, add a few tablespoons of mineral oil after refilling; it floats on the water and slows evaporation for months.

If the odor persists after refilling, the trap is not the only suspect. A failed toilet wax ring, a cracked drain line, or a blocked vent can all leak gas too. Work through the full list in our guide to track down a sewer smell in the house.

Frequently asked questions

What does a P-trap do?

A P-trap holds a small pool of standing water, called the trap seal, in the U-bend under a fixture. That water plug blocks sewer gas from rising up the drain into the room while still letting waste water pass through. Every sink, tub, and shower needs one to keep drain odors and sewer gases out of the house.

How much water should a P-trap hold?

Both the IPC and UPC require a trap seal between 2 and 4 inches deep, measured from the top of the dip to the top of the outlet weir. Less than 2 inches and a draining fixture can siphon it dry. More than 4 inches and the deep leg fouls with solids instead of self-scouring. Standard traps sit in that range by design.

Why are S-traps not allowed by code?

An S-trap drops straight down after the U-bend with no horizontal run and no vent, so a full basin of draining water pulls a vacuum behind it and siphons the trap seal out. The empty trap then lets sewer gas in. Both the IPC and UPC prohibit S-traps in new work and require a vented P-trap instead.

What size P-trap do I need?

A bathroom lavatory sink uses a 1-1/4 inch P-trap. Kitchen sinks, laundry tubs, and bathtubs step up to 1-1/2 inch because they move greasy water, food scraps, and larger surges. Showers typically run a 2 inch trap. Match the trap to the drain outlet and tailpiece so the slip nuts seal without adapters.

Does every drain need a P-trap?

Nearly every plumbing fixture must be separately trapped and vented, and toilets are the main exception because they have a built-in trap in the porcelain. Sinks, tubs, showers, floor drains, and washing machine standpipes all need a P-trap. Codes prohibit stacking two traps on one fixture, which can lock air between them and slow the drain.

Why does my P-trap smell, and how do I fix it?

A drain that sits unused for weeks lets the trap water evaporate, breaking the seal so sewer gas flows straight in. Run the tap for one to two minutes, or pour about a half gallon of water down the drain, to refill the trap. For a rarely used drain, add a few tablespoons of mineral oil to slow evaporation.

Last reviewed: September 2026.