What Size P-Trap Do You Need? Trap Sizes by Fixture

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Quick answer from a licensed plumber: A bathroom sink takes a 1¼” trap. Almost everything else in the house — kitchen sink, bathtub, shower, laundry tub — takes 1½”. And there is one rule that catches people out: a trap must not be larger than the pipe it drains into. Bigger is not safer here.

Minimum trap size by fixture

Those are the values printed in the code’s trap-sizing table (Table P3201.7 as adopted). They are minimums — the smallest you are permitted to use, not a target to beat.

📌 Honest note on the citation: we read this table in an adopted IRC-family residential code. Codes are adopted and amended locally, and table numbers move between editions and between the IRC/IPC and UPC families. Treat these as the standard values and confirm your local adoption before a permitted job.

⛔ The rule that surprises people: bigger is not better

The code is explicit: “A trap shall not be larger than the drainage pipe into which the trap discharges.”

This runs against instinct. Plenty of people reason that a bigger trap drains faster and cannot hurt. It can, for two reasons:

• You would be draining a large pipe into a smaller one, which creates a deliberate restriction at exactly the point debris collects.
• An oversized trap does not scour. A drain line clears itself because the flow fills the pipe and carries solids along. Oversize it and the same volume of water becomes a shallow trickle across the bottom, which drops grease and sediment instead of sweeping it out. You get a trap that needs cleaning constantly and never quite runs right.

That second point is the one worth internalizing: drainage is sized to keep the water moving fast enough, not to give it the most room. It is the same principle behind the slope limits on a trap arm.

The bathroom-sink exception, and the adapter that causes leaks

The bathroom sink is the one fixture on the list at 1¼”, and that single difference is behind a lot of under-sink confusion.

What actually happens: someone buys a “sink drain kit” without checking, gets 1½” parts, and ends up stacking reducing washers and adapters to make it meet a 1¼” tailpiece. Every one of those extra slip joints is a place to weep. If your cabinet is damp, that stack is worth looking at before anything else — see a sink leaking underneath.

Measure before you buy. Measure the outside diameter of the tailpiece coming down from the sink, not the hole in the cabinet and not the pipe in the wall. Kits are sold for both sizes and they are not interchangeable.

Why kitchen sinks get the bigger trap

It is not about the volume of water. It is about what is in it.

A bathroom sink handles soap and toothpaste. A kitchen sink handles grease, coffee grounds, starch and food solids — the things that build a lining on the inside of a pipe over years. The extra quarter inch of diameter is margin against that lining, which is also why a kitchen drain is the one that eventually needs clearing. If yours is already slowing, why a sink drains slowly covers the sequence to work through.

A garbage disposal does not change the trap size requirement, but it does change what arrives at the trap. Disposals shred food fine enough to travel and then settle, which is why a disposal on a marginal drain line often makes a slow drain noticeably worse rather than better.

Buying the right parts

• Match the tailpiece, measured, not assumed.
• Do not oversize to “improve” flow — the code forbids it and the physics does not support it.
• Buy a kit rather than loose parts where you can. A kit arrives with matched washers, which is the single most common cause of a weeping slip joint.
• Never wrench-tighten plastic slip nuts. Hand-tight plus a quarter turn. Over-tightening cracks the nut, and the crack is usually invisible until it drips.

Our guide to P-trap replacement kits covers what comes in each and which size to pick.

FAQ

What size P-trap does a bathroom sink use?

1¼ inches. The code’s trap-sizing table lists a lavatory at a 1¼” minimum, and it is the only common household fixture at that size — kitchen sinks, bathtubs, showers and laundry tubs are all 1½”.

What size P-trap does a kitchen sink use?

1½ inches, per the same table. The larger size is about what a kitchen drain carries — grease, starch and food solids that line the pipe over time — rather than about the volume of water.

Can I use a bigger trap than required?

No. The code states that “a trap shall not be larger than the drainage pipe into which the trap discharges.” Beyond the code issue, an oversized trap drains worse in practice: the same water becomes a shallow trickle instead of a full-bore flow, so it stops scouring and starts dropping grease and sediment.

What size trap does a shower need?

1½ inches for a shower rated at 5.7 gpm and less, which covers standard residential showers. Bathtubs and laundry tubs are also 1½”.

How do I know which size I have?

Measure the outside diameter of the tailpiece — the pipe dropping down from the sink drain — rather than guessing from the cabinet opening or the pipe in the wall. Kits are sold for both sizes and mixing them means stacking adapters, which adds slip joints and places to leak.

Sources

• UpCodes — Fixture Traps, Table P3201.7 as adopted: Lavatory 1¼” · Kitchen sink 1½” · Bathtub 1½” · Shower (5.7 gpm and less) 1½” · Laundry tub 1½”. Governing text: “A trap shall not be larger than the drainage pipe into which the trap discharges.”

Note on jurisdiction: read in an adopted IRC-family residential code. Table numbers and values vary between code editions and between the IRC/IPC and UPC families — confirm your local adoption before relying on a figure for permitted work.

Related: getting the size right is half of it; the material matters too — see PVC vs ABS pipe.

Related: once the size is right, the joint still has to be made properly — see is purple primer required.