Walk into the plumbing aisle and you will find P-traps in 1 1/4 inch and 1 1/2 inch sizes sitting side by side, and they look almost identical. For a bathroom sink the right one depends on your tailpiece, your wall drain and your local code. Buy the wrong size and nothing seals. Here is how to work out which trap you need, how the pieces fit together, and a quick way to measure before you leave for the store.
The short answer
Most bathroom sinks, also called lavatories, use a 1 1/4 inch tailpiece and a 1 1/4 inch trap. Codes allow that because a lavatory drains slowly compared with a kitchen sink or tub. Many plumbers still prefer a 1 1/2 inch trap with a reducing washer at the top, because it clears hair and soap better and matches the 1 1/2 inch drain line most homes have in the wall.
Both work. What matters is that each joint uses the matching nut and washer, and that the trap arm connects cleanly to the wall stub.
Common sizes compared
| Fixture | Typical tailpiece | Minimum trap size (IPC) | Common install |
|---|---|---|---|
| Bathroom sink (lavatory) | 1 1/4 in | 1 1/4 in | 1 1/4 in, or 1 1/2 in with reducer |
| Pedestal sink | 1 1/4 in | 1 1/4 in | Chrome 1 1/4 in for looks |
| Vessel sink | 1 1/4 in or 1 1/2 in | 1 1/4 in | Matches drain kit |
| Kitchen sink | 1 1/2 in | 1 1/2 in | 1 1/2 in |
| Shower | n/a | 2 in | 2 in |
| Bathtub | n/a | 1 1/2 in | 1 1/2 in |
Most trap kits cost about $8 to $20 in plastic and $25 to $60 in chrome. A reducing washer, if your kit does not include one, is usually under $3.
How to measure your sink drain step by step
You do not need special tools. A tape measure or a piece of string and a ruler will do.
- Look under the sink and find the vertical tailpiece coming down from the drain.
- Measure across the outside of the tailpiece. About 1 1/4 inches means a 1 1/4 inch tailpiece. About 1 1/2 inches means 1 1/2 inch.
- If you cannot get a tape around it, wrap string around the pipe, mark it and measure the length. A circumference of about 4 inches means 1 1/4 inch, and about 4 3/4 inches means 1 1/2 inch.
- Check the wall stub. Most have a trap adapter with a slip nut. Look for size markings on the nut or measure the pipe that comes out of the wall.
- Note the distance from the tailpiece center to the wall, so you know whether the trap arm needs trimming.
- Take the old trap with you to the store if you can. Matching parts side by side saves a second trip.
Mixing sizes the right way
Going from a 1 1/4 inch tailpiece to a 1 1/2 inch trap is common and easy. Buy a 1 1/2 inch trap kit with a reducing washer, or a separate 1 1/2 by 1 1/4 inch reducing washer, and use it on the top slip nut. The trap arm then runs at 1 1/2 inches into the wall.
What you should avoid is the reverse, going from a larger pipe down into a smaller one in the direction of flow. Water should never step down in size as it heads toward the sewer.
If your trap is the right size but still gives trouble, see our guides to a P-trap leaking at the slip nut or a clogged P-trap. And if the sink drain itself needs replacing, our walkthrough on how to remove a bathroom sink drain flange is a good place to start.
Other things that affect the fit
- Trap washers: slip joint washers come in flat and beveled shapes and in plastic or rubber. Our guide to P-trap washer types explains which goes where.
- Wall stub height: the trap outlet must be at or above the stub. If it is lower, read about a P-trap lower than the drain pipe.
- Depth of the trap seal: factory traps hold the right 2 to 4 inch seal. See how deep a P-trap should be.
- Cramped vanities: drawers can crowd the trap. An offset P-trap can help.
Safety and code notes
The IPC and UPC set 1 1/4 inch as the minimum trap size for a lavatory. Your local code may differ slightly, and a few jurisdictions require 1 1/2 inch for all new work. Do not use corrugated flex traps where an inspection is involved, since many inspectors reject them. Plastic slip nuts should be tightened by hand plus a small turn, because overtightening cracks them. Wear gloves when you remove an old trap, and keep a bucket handy.
Frequently asked questions
What size P-trap do I need for a bathroom sink?
Usually 1 1/4 inch, matching the tailpiece. A 1 1/2 inch trap with a reducing washer also works and drains a little better.
Can I use a 1 1/2 inch trap on a 1 1/4 inch tailpiece?
Yes. Use a 1 1/2 by 1 1/4 inch reducing washer on the top slip nut.
Are bathroom and kitchen P-traps the same size?
Not always. Kitchen sinks use 1 1/2 inch, while bathroom sinks often use 1 1/4 inch.
How do I know my drain pipe size without removing it?
Wrap a string around it and measure. About 4 inches around is 1 1/4 inch and about 4 3/4 inches is 1 1/2 inch.
Is a chrome trap better than plastic?
Chrome looks nicer under a pedestal sink, but plastic is cheaper, does not corrode and is easier to install.