Quick answer from a licensed plumber: A bathroom sink takes a 1¼” trap. Almost everything else in the house — kitchen sink, bathtub, laundry tub — takes 1½”. The shower is the exception worth knowing: 1½” under the IRC, but 2″ under the UPC, so check which code your area has adopted before you buy. 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. If it is a sink trap you are replacing, our P-trap replacement guide has picks for both 1¼” and 1½” drains.
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Minimum trap size by fixture
| Fixture | Minimum trap size |
|---|---|
| Lavatory (bathroom sink) | 1¼” |
| Kitchen sink | 1½” |
| Bathtub | 1½” |
| Shower (5.7 gpm and less) | 1½” (IRC) · 2″ under the UPC |
| Laundry tub | 1½” |
| Clothes washer standpipe | 2″ |
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.
Where the two code families disagree: the shower. The IRC table above allows 1½” for a shower at 5.7 gpm and less. The UPC’s own fixture table lists a single-head shower at a 2″ minimum trap and trap arm, and permits 1½” only on a bathtub-to-shower retrofit within a 36″ × 60″ footprint. Every other fixture on the list above is the same size in both families — the shower is the only one that changes.
📌 Honest note on the citation: we read the IRC table (P3201.7) in the Texas Windstorm Insurance Association Residential Code 2024 and the UPC table (702.1) in the Hawaii Plumbing Code 2021, both as published on UpCodes. 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.
P-Trap Size for a Lavatory: The Bathroom-Sink Exception
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.
Kitchen Sink P-Trap Size: Why It Is Bigger
The size comes from the code table. What differs in practice is what goes down the drain.
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. That lining is 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 a trap larger than the pipe it drains into, and the physics does not support it.
- Buy a kit rather than loose parts where you can. A kit arrives with matched washers, and a mismatched washer is a 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.
If the trap under a bathroom sink is the 1¼” one and you would rather have metal than plastic, the PROFLO PFPTB100 17-gauge chrome P-trap, 1-1/4″ x 1-1/4″ is the size the trap table actually asks for at a lavatory. The listing gives it as a 17-gauge brass P-trap with 1-1/4″ x 1-1/4″ slip-joint tubular connections and a high box flange, so it meets a 1-1/4″ lavatory tailpiece with no reducing washer. Most kits on the shelf are 1½”, which is where the adapter stack starts.
Bathtub and Tub P-Trap Size
A bathtub takes a 1½” trap, the same as a kitchen sink or a laundry tub, and both code families agree on it. The size is rarely the problem with a tub: the trap sits under the floor and is reached through an access panel or from the ceiling below, so the work is getting to it rather than choosing it.
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 and laundry tubs are all 1½”, and a shower is 1½” under the IRC but 2″ under the UPC.
What size P-trap does a kitchen sink use?
1½ inches, per the same table. A kitchen drain also carries grease, starch and food solids that line the pipe over time.
Can I use a bigger trap than required?
Not larger than the drain pipe it connects to. The table sizes are minimums, and 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?
It depends which code your area has adopted. The IRC allows 1½ inches for a shower rated at 5.7 gpm and less, which covers standard residential showers. The UPC requires 2 inches for a single-head shower and permits 1½ inches only on a bathtub-to-shower retrofit within a 36″ × 60″ footprint. Bathtubs and laundry tubs are 1½” in both.
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: Table P3201.7 was read in the Texas Windstorm Insurance Association Residential Code 2024, an adoption of the 2024 IRC published on UpCodes. The UPC shower figures are from Table 702.1 of the Hawaii Plumbing Code 2021, an adoption of the 2021 UPC published on UpCodes: single-head shower trap 2 inches, with 1-1/2 inches permitted for a bathtub to shower retrofit up to 36 inches wide and 60 inches long. 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.