Are All Bathroom Sink Drain Pipes the Same Size?

Drain parts that look the same on the shelf don't fit each other. A 1-1/4 inch trap won't thread onto a 1-1/2 inch tailpiece, and the mismatch usually shows up only once the old parts are off. Knowing which size belongs to which part saves a second trip to the store.

Tailpiece, trap and wall drain are three different pieces

"Drain pipe" covers several parts, and each has its own size.

Delta sells a 1-1/4 inch diameter tailpiece extension tube that "extends lavatory sink tailpiece to waste trap" and can be cut to length. Its decorative P-trap is listed with a 1-1/4"-11 NPSM threaded connection to the drain body or tailpiece. That's the standard bathroom sink pairing.

The wall drain is the odd one. The trap size is set by code and by the sink. The stub-out size was set by whoever roughed in the plumbing, so it doesn't always match. When it differs, an adapter or reducing washer bridges the gap, and plumbing aisles stock them for exactly this.

What the plumbing code sets

The Uniform Plumbing Code (UPC) lists a minimum trap size for each fixture in Table 702.1. King County, Washington reproduces the values, and the Minnesota rules adopt the same table:

The kitchen sink row carries a footnote: provide a 2-inch minimum branch drain beyond the trap arm. A fixture unit is the code's measure of how much load a fixture puts on the drainage system. One bathroom sink counts as 1.

The International Plumbing Code (IPC), used in many other states, has its own Table 709.1(1). I couldn't read the lavatory row cleanly from a fetched copy, so I'm not quoting an IPC number here. Which code applies depends on your state and city, and local amendments change things. If you're replacing parts in the same size you already have, code rarely comes into it. If you're changing the layout or roughing in new plumbing, ask the local building department which edition it enforces.

How long the trap arm can run

Codes limit how far a trap can sit from its vent, so a sink can't be moved a long way from the wall without a new vent. A city of Richfield, Minnesota plumbing handout gives maximum trap arm lengths of 42 inches for 1-1/2 inch pipe and 60 inches for 2 inch pipe. Smaller pipe has a shorter limit than 42 inches. In the UPC the limit comes from Table 1002.2, which Minnesota's rule cites. The IPC has its own version, so check the table your jurisdiction uses before relocating a sink.

How to measure what you have

Do this before shopping, with the old parts in hand.

1. Put a bucket under the trap and unscrew the slip nuts by hand or with slip-joint pliers.
2. Measure the outside diameter of the tailpiece. About 1.25 inches is a 1-1/4 inch part.
3. Measure the trap and trap arm the same way.
4. Look into the wall opening and measure its inside diameter. If it's about 2 inches across, it often needs a trap adapter that reduces it to a smaller trap arm.
5. Note the pop-up style and whether the thread is fine or coarse. Delta lists its trap on a 1-1/4"-11 NPSM thread, and parts from other makers may not mate with it.

Take the old parts or a clear photo to the store. Bring the sink's model number too, since the drain body that comes with a faucet or vessel sink can have a different tailpiece than a standard one.

Fitting the parts together

Delta's own product notes say the tailpiece length may need to be cut to fit, and that for a compression fit the tailpiece must go at least 1 inch inside the trap body (a point from Delta's Q&A, not a code rule). Dry-fit everything before tightening the nuts (my advice, not Delta's).

A kitchen-size 1-1/2 inch plastic trap such as Oatey's Dearborn model has an 8 inch straight tube and is sold for tubular drain applications. That's a larger part than a bathroom sink needs. If a store kit looks oversized, check the label for the pipe size before opening it.

Frequently Asked Questions

Are all bathroom sink drain pipes the same size?

No. Most are 1-1/4 inch at the tailpiece and trap, but the wall drain behind it is often 1-1/2 inch or 2 inch. Kitchen sinks, tubs and showers use bigger traps.

What size is a standard bathroom sink drain?

1-1/4 inch is the standard for a single bathroom sink. The UPC lists 1-1/4 inch as the minimum trap size for a single lavatory.

Can I connect a 1-1/4 inch tailpiece to a 1-1/2 inch trap?

Yes, with a reducing washer or an adapter made for it. Don't force mismatched parts together. Check that the new part is rated for the connection.

What size is the pipe in the wall for a bathroom sink?

It varies, and there's no single answer. Many are 1-1/2 inch and some are 2 inch. Measure the opening, and use a trap adapter if the stub-out is larger than the trap arm.

What size trap does a double vanity need?

The Minnesota version of the UPC table lists 1-1/2 inch for lavatories in sets of two or three, counting 2 fixture units. Check the table your building department enforces.

Sources

• King County: Drainage and venting fixture unit values (UPC Table 702.1 trap sizes for lavatory, tub, kitchen sink, shower, washer)
• Minnesota Rules 4714.0702: Fixture Unit Equivalents (UPC table as adopted in Minnesota, single lavatory 1-1/4 inch, lavatory sets 1-1/2 inch)
• Minnesota Rules 4714.1002: Traps and trap arm vent distance (the trap arm must be within the distance in Table 1002.2)
• City of Richfield, MN: Basic Plumbing handout (trap arm definition and maximum lengths)
• Delta Faucet: Lavatory tailpiece extension tube (1-1/4 inch diameter tailpiece extension, can be cut to length)
• Delta Faucet: DT041141 bottle trap (1-1/4"-11 NPSM connection, insertion depth note in Q&A)
• Oatey: Dearborn 1-1/2 in. tubular P-trap (1-1/2 inch pipe size and 8 inch straight tube)