There’s no single shower drain size. Pennsylvania’s residential code text sets the drain outlet at not less than one and a half inches in diameter. Then it sizes the trap by the total flow through the showerheads and body sprays. The more water the fixtures can run at once, the larger the trap. So the fixture list decides, and it’s worth settling with your designer before the shower floor is built.

The drain and the trap sit under the shower floor. Their size follows the fixtures the shower can run. So the fixture list and the drain are easier to settle on paper than after the floor is built.

What Sets Shower Drain Size?

The fixtures set it. The code text puts a floor under the drain outlet, then sizes the trap by how much water can flow through the shower at one time. There’s no one number that fits every home, because fixture lists aren’t the same from home to home.

The plumbing fixtures chapter of Pennsylvania’s 2021 residential code, as published on UpCodes says shower drains need an outlet of not less than one and a half inches in diameter. That’s a floor. It tells you the least the outlet can be, and it stops there.

The outlet settles one piece of the shower. What it doesn’t settle is the trap below it, or the pipe that trap drains into. Those follow water. And the water comes from the fixtures you choose, which is why flow drives the rest of the sizing.

How Does the Code Size the Trap?

By flow. The code text sizes a shower trap off the total flow rate through the showerheads and body sprays, in bands. The first band sets the trap at one and a half inches. Each band above it calls for a larger trap as the total climbs. Add fixtures, and the shower can cross into the next band.

Those sizes sit in a table in the code’s chapter on traps. The table reads the total flow through the showerheads and body sprays, then gives a minimum trap size for each band of flow.

  • 5.7 gallons per minute and less: a trap of one and a half inches.
  • More than 5.7 and up to 12.3 gallons per minute: a trap of 2 inches.
  • More than 12.3 and up to 25.8 gallons per minute: a trap of 3 inches.
  • More than 25.8 and up to 55.6 gallons per minute: a trap of 4 inches.

Those are the code text’s minimum trap sizes. The same section adds that a trap can’t be larger than the drainage pipe it discharges into. So shower drain pipe size has to keep up. The pipe behind the trap has to be at least as large as the trap. Your plumber confirms the size on the job against the code edition the local building office enforces.

Why Count Every Showerhead and Body Spray?

Because the table reads a total, not a single fixture. Every head, hand shower and body spray that can run at the same time adds to that number. A longer list can push the same shower into the next band up.

Take a hypothetical shower. Say three fixtures each show a rated flow of 2 gallons per minute on their spec sheets. Together that’s 6 gallons per minute. That total sits past the 5.7 line, so the table moves to the next band. The figures are made up, not a claim about any real fixture. The arithmetic is the point.

So read the rated flow printed on each fixture’s spec sheet, and write it beside the fixture on your list. Then ask your designer which fixtures can run at the same time. For example, ask whether a hand shower and a head can run at the same time. That answer changes the total the table reads.

Why Settle the Drain Before the Floor Is Built?

Because the drain and the trap sit under the shower floor. While the plan is on paper and the floor is still open, a larger trap and drain can be planned in. Once the floor is built over them, changing that size may mean opening it back up. The fixture list can answer that question early.

Lang’s post on how a curbless shower gets built tells readers to confirm that slope, drainage and waterproofing work together as one system. Drain size is one piece of that drainage question.

An older Lang’s post on what to ask about plumbing before a bathroom remodel calls a remodel the time to update supply and drain pipes if necessary, and it flags supply pipes as more critical when a shower will have multiple shower heads or body sprays. Supply is one side of the question. The drain is the other side.

What Should You Bring to a Design Consultation?

Bring the fixture list. Write down every showerhead, hand shower and body spray you want in the shower. Bring the spec sheet for each one, since that’s where the rated flow is printed. Photos of the existing shower help too, along with any drawings you already have.

With that list in hand, a designer can put the fixtures on the plan instead of guessing at them. Lang’s follows a 12-step process for renovation, and a jobsite visit sits inside it. On that visit, Lang’s designers perform a complete in-home analysis, which is a chance to compare the plan with the room.

Bathroom designs vary with which bathroom you’re remodeling and who will use it. So the fixture list belongs in the design conversation early. Lang’s bathroom remodeling team can help design the space you want, and you can start that conversation through bathroom remodeling with Lang’s designers.

Frequently Asked Questions About Shower Drain Size

Is there one shower drain size for every home?

No. The code text sets a one-and-a-half-inch floor for the drain outlet, then sizes the trap by the total flow through the showerheads and body sprays. So the fixture list decides, and a shower with one head and a shower running several outlets may not land in the same band. Ask your designer to confirm the size against your list.

Does a body spray change the drain size?

It can. The table counts the total flow through showerheads and body sprays together, so adding a spray raises the total. If that total crosses a band line, the trap size moves up with it. Your plumber confirms the final size on the job, so it’s worth asking your designer before the fixtures are chosen.

Is a shower drain the same size as a bathtub drain?

They share a floor, but they don’t have to match. The code text sets a waste outlet of not less than one and a half inches for a bathtub, and the same one-and-a-half-inch minimum for a shower drain outlet. A bathtub trap, with or without a shower head, has a minimum of one and a half inches. A shower’s trap can be larger, because it follows the flow through the fixtures.

Can the drain size change after the floor is built?

It may be possible, but the drain and the trap sit under the shower floor. A larger size can mean reopening that floor to reach them. That’s the practical reason to settle the fixture list while the plan is still on paper. Ask your designer and your plumber how the size will be confirmed before the floor goes in.

Who can help me plan the fixture list?

Lang’s design team. Design consultations are available in the showroom and online by appointment. Lang’s follows a 12-step remodeling process that includes a jobsite in-home analysis by its designers, and a later stage where the design team finalizes all dimensions, floor plans and mechanical specifications. Bring the list and the spec sheets, and you’re working from the rated flow instead of a guess.

Bring Your Shower Fixture List to a Design Consultation

In Lang’s 12-step remodeling process, designers visit the jobsite for a complete in-home analysis before the design team finalizes all dimensions, floor plans and mechanical specifications.

That order is worth keeping in mind for a shower. The fixture list and the drain are easiest to settle while the floor is still open and the drawings are still open.

The design showroom is in Newtown, and design consultations are available in the showroom and online by appointment. Lang’s 12-step remodeling process lays out the stages a renovation moves through.

So bring the fixture list and the spec sheets. Talk with Lang’s design team about how the shower drain size will be confirmed against that total before the shower floor is built.