What Adapter For
Your answer

What size P-trap do I need?

A bathroom sink takes a 1-1/4" P-trap. A kitchen sink, bathtub or laundry tub takes 1-1/2", and a washing machine standpipe 2".

What’s the trap for?

Bathroom sinkA lavatory or vanity basinKitchen sinkWith or without a disposer or dishwasherBathtubWith or without a shower headShowerA shower stall or walk-in showerWashing machineThe standpipe its hose drains intoLaundry tubOne or two compartmentsToiletAny floor or wall toiletSomething elseA fixture not listed here

Trap size

1-1/4" P-trap

The code minimum for a bathroom sink, and the usual size.

On the package: 1-1/4 in. tubular P-trap

Available on Amazon

1-1/4" P-trap kit
  • Most bathroom sinks drain through a 1-1/4" tailpiece.
  • A 1-1/2" trap fits too, with a 1-1/4" x 1-1/2" reducing washer (many come with one).
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Trap size

1-1/2" P-trap

The minimum for a kitchen sink, with or without a disposer or dishwasher.

On the package: 1-1/2 in. tubular P-trap

Available on Amazon

1-1/2" P-trap kit
  • Kitchen sinks take 1-1/2" tubular parts only. 1-1/4" parts are too small.
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Trap size

1-1/2" trap

The minimum for a bathtub, whether or not it has a shower head.

On the package: 1-1/2 in. P-trap

Available on Amazon

1-1/2" tub P-trap
  • It can’t be larger than the drain pipe it empties into.
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Trap size

1-1/2" or 2", by your code

The IPC and IRC set 1-1/2" for a home shower (5.7 gallons per minute or less in all). Areas on the UPC set 2".

  • Match the trap to the shower drain’s outlet. It can’t be smaller than your code’s minimum or larger than the pipe it drains into.
  • Your local building department can tell you which code applies.

Under the IPC and IRC, several heads or body sprays over 5.7 gpm need a 2" trap, which covers up to 12.3 gpm.

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Trap size

2" trap

The minimum for a washing machine standpipe and its trap.

On the package: 2 in. P-trap

Available on Amazon

2" P-trap
  • The standpipe rises 18" to 42" above the trap’s weir, the level water spills over.
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Trap size

1-1/2" laundry tub trap

The minimum for a laundry tub with one or two compartments.

On the package: 1-1/2 in. P-trap

Available on Amazon

1-1/2" P-trap kit
  • If it’s a thin-walled tubular trap, it joins with slip nuts and washers. Plastic tubular parts aren’t glued.
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Trap size

None, because it’s built in

A toilet’s trap (its trapway) is cast into the bowl. A P-trap below it would double-trap it, which code forbids.

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Trap size

The size of its drain outlet

For a fixture the code tables don’t list, the trap matches the drain outlet and is never under 1-1/4".

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A shower is 1-1/2" or 2" depending on which plumbing code your area uses. A toilet needs none: its trap is built into the bowl.

P-trap size chart

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P-trap sizes

Minimum P-trap size by fixture (US)
FixtureIPC and IRCUPC
Bathroom sink (lavatory)1-1/4"1-1/4"
Kitchen sink1-1/2"1-1/2"
Bathtub1-1/2"1-1/2"
Shower1-1/2" (5.7 gpm or less)2"
Laundry tub1-1/2"1-1/2"
Washing machine standpipe2"2"
ToiletBuilt inBuilt in

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These are the smallest traps the US plumbing codes allow. Each area adopts either the International Plumbing Code (IPC) and International Residential Code (IRC), or the Uniform Plumbing Code (UPC). They agree on everything but the shower.

A trap can be bigger than the minimum, but never bigger than the drain pipe it empties into. Under the IRC, a fixture whose tailpiece is larger than the chart’s size gets a trap the same size as the tailpiece.

Why a 1-1/2" trap won’t slip onto 1-1/2" pipe

Sink traps are tubular: thin-walled parts joined with slip nuts and washers, sized by their outside diameter: a 1-1/2" tubular trap fits drain pipe 1-1/2" across the outside. The drain pipe in the wall is usually schedule 40, named by a nominal size: 1-1/2" schedule 40 is 1.900" across, and 2" is 2.375".

So the two don’t meet directly. A trap adapter joins them: a slip joint for the trap on one end, glued onto 1-1/2" schedule 40 on the other. Many trap kits include one.

Putting one in

  1. Slide a slip nut, then a slip washer, onto the tailpiece and onto the wall tube before you fit the trap.
  2. A slip joint into the wall tube needs a washer. A tapered plastic (ground) joint doesn’t.
  3. Don’t glue plastic tubular parts: they’re polypropylene, which solvent cement won’t join. The slip nuts and washers make the seal.
  4. Tighten every slip nut fully, then run water and check each joint for drips.

Code rules for traps

UK and Australia

In England, Approved Document H sets these minimum trap sizes and water seals (Wales, Scotland and Northern Ireland have their own rules):

Minimum trap sizes, England
ApplianceTrapSeal
Washbasin, bidet32 mm75 mm
Bath, shower40 mm50 mm
Kitchen sink, washing machine, dishwasher40 mm75 mm
WCBuilt into the pan50 mm

In Australia, a bath or an individual shower drains through a trap and waste pipe of at least DN 40 (AS/NZS 3500.2).

Connecting the water to the sink as well?

Faucet supply line size

Common questions

What size P-trap goes on a bathroom sink?

1-1/4 inch is the code minimum and the usual size. A 1-1/2 inch trap also works if you add a reducing washer on the tailpiece.

Does a toilet need a P-trap?

No. The bowl has a trap cast into it, and adding a second trap below would break the codes' rule against double trapping.

Is a shower trap 1-1/2 or 2 inch?

It depends on your area's code. Under the IPC and IRC, an ordinary home shower needs 1-1/2 inch; the UPC calls for 2 inch. Your local building department can tell you which applies.

What size P-trap is in mm?

US sink traps are 1-1/4 inch (about 32 mm) and 1-1/2 inch (about 38 mm). In England, basins use 32 mm traps and sinks, baths and showers 40 mm.

Keep going

Faucet supply line sizeShower head thread sizeToilet flapper sizeIdentify it

Where we get our information from

The sizes on this page come from published standards and the makers’ own documents. Checked against published data from Oatey, International Code Council, Public Health Seattle and King County, Charlotte Pipe and Kohler (plus 2 more).

See all 15 sources
  • International Plumbing Code 2021 and 2024, Table 709.1 (minimum trap sizes) and sections 406.2, 709.2, 802.4.3 (as adopted in South Carolina)
  • International Plumbing Code 2021, Chapter 10, sections 1002.1-1002.6 (fixture traps, slip joints, prohibited traps, trap seals, trap size)
  • International Residential Code 2021, Chapter 32, Table P3201.7 and sections P3201.2, P3201.5-P3201.7 (as adopted in South Carolina and Virginia)
  • International Residential Code 2021, sections P2703.1 (tailpieces), P2704.1 (slip joints), P2708.2 (shower drains)
  • Public Health Seattle and King County, Drainage and venting fixture unit values (excerpt from Uniform Plumbing Code Table 702.1)
  • Oatey, Brass Tubular frequently asked questions: 1-1/4" and 1-1/2" tubular, slip nuts and washers, S-traps
  • Dearborn Brass (Oatey), Plastic Tubular 1-1/2" P-Trap with PVC Quarter Bend submittal: 1-1/4" x 1-1/2" reducing washer, ASTM F409
  • Dearborn (Oatey), Plastic 1-1/4" Tubular P-Traps product page
  • Oatey, Plastic Trap Adapter HDC9000A and 1-1/2" P-Traps with Threaded Adapter: fits 1-1/2" schedule 40, reducing washer
  • Oatey, Insta-Plumb 1-1/4" and 1-1/2" P-Traps: fit 1-1/4" and 1-1/2" O.D. drain pipe; Bathroom P-Trap instruction guide
  • Charlotte Pipe, Submittal Package, PVC Schedule 40 DWV System (2026): average outside diameters
  • Kohler, Highline toilet K-3999 specification sheet (US/CA): 2-1/8" glazed trapway, standard 12" rough-in from the finished wall to the outlet center
  • HM Government, Approved Document H (2015), Table 1: minimum trap sizes and seal depths
  • NSW Government, Floor waste and fixture requirements (quoting AS/NZS 3500.2:2021)
  • Oatey, Instruction Guide, Bathroom P-Trap

General guidance from manufacturer and standards data. Parts vary by brand, so check the size printed on the package against what you measured. For gas lines or anything inside a wall, use a licensed plumber.