Need a drain for your new tub or bathroom remodel? Learn how (and what) to measure to make sure you choose the right size.

How to Size a Tub Drain

Bought a new tub? Remodeling a bathroom, or building a new one? There's a good chance you'll need a new drain and overflow assembly (aka waste and overflow) to complete the project. Whether you're simply matching the finish in a new color scheme, or need to accommodate an entirely new tub, you'll have to do a bit of measuring to ensure you select the right kit. This could be as simple as measuring the old assembly, but when that's not possible, you'll have to go to the tub itself.

What Is a Drain/Waste & Overflow Assembly?

Every tub has drainage tubing that connects to the home's drain pipes below, and most tubs have an overflow - that circular plate below the lip of the front wall of the tub (that sometimes has a trip lever). With a freestanding tub, the tubing that connects the overflow and the tub drain to the waste pipes below is exposed; built-in alcove tubs have all of this neatly hidden.

Tub overflow diagram

While we all know what the drain is for, the overflow is a bit more mysterious for some. It's essentially just another hole in the tub, covered by a faceplate, through which water can exit. It's intended to prevent flooding, though there is some debate as to its actual efficacy - with the faceplate attached the opening is rather small, casting doubt on its ability to remove water fast enough to keep up with a flowing tub spout (our advice: never leave a filling tub unattended).

Though most tubs sold in the US include an overflow hole, don't panic if yours doesn't - some municipal plumbing codes do not require one. If you're looking to get a tub without an overflow, be sure to check with local code authorities before proceeding, lest you fail inspection.

Pro Tip: Drain and overflow kits are typically made of brass, PVC, ABS or PP (Polypropylene). Of these, brass is the most durable and attractive, but will cost more. Aside from this, no material is really much better than another for the purpose being served. Beware, though: local code may dictate what can be installed in your city or county. While your plumber should be aware of any applicable regulations, DIYers should check before purchasing online.

Measuring for a new drain/overflow kit is fairly simple - you'll only need 2-3 measurements to start shopping. And since most kits are sized to fit a range of tub sizes, finding a compatible assembly is made that much easier. If you're unfamiliar with the drain/overflow assembly or its terminology, the diagram above lays it all out.

Helpful Hint: Have you checked your tub's paperwork? The manufacturer may have included sizing information for the drain and overflow assembly - no measuring necessary!

How & What to Measure


A freestanding tub makes measuring easy, since everything is exposed. Built-in tubs can present a bit of a challenge, however. A properly-constructed bathroom will have an access panel in the alcove (if applicable), or the wall adjacent to the built-in tub's plumbing (sometimes accessible from the next room or a closet). If the location of this opening isn't obvious, try tapping any wood or tile that lies directly behind the front of the tub, or to the side - access panels can be expertly concealed and you'll need to listen for a hollow space.

Without this convenient access point, the only way to get to the drain/overflow plumbing (and the relevant parts of the tub) is by opening the wall or floor, or finding the right spot under the house - all of which may require the aid of a professional. Although access panels aren't required by code in every situation, we believe they're essential. Installing one will save you and future homeowners/plumbers a lot of hassle.

Pro Tip: It's not uncommon to have a tub installation that makes connecting the drain/overflow assembly to the bathroom drain line incredibly difficult (if not impossible) when using straight, rigid plastic or brass. Likewise, there are plenty of uniquely-shaped tubs out there that can make things difficult. For these situations, a flexible drain assembly will be your salvation!

Overflow Depth:

Measure straight down from the center of the overflow to the point at which the drain is mounted to the tub: depending on what's easiest to get to, you can measure from outside the tub down to its underside, or from inside the tub to the tub floor. Do not follow the curve of the tub; measure straight down.

Drain Shoe Length:

As seen in the diagram above, the drain shoe is an elbow that lies directly below the tub drain, directing water from the tub into the horizontal pipe that leads to the bathroom drain line below the floor. You'll need to measure from the center of the drain to the "run" of the waste tee - where the overflow tube fits into it.

Tub Depth/Height:

Measure from the bathroom floor to the top rim of the tub. Tub depths can range from 14" for an alcove tub, to well over 20" for some freestanding tubs. This measurement probably won't come into play when selecting your assembly, but could be a factor depending on how the kits you're looking at are sized.

With these measurements in hand, you're ready to find the perfect drain and overflow assembly!

Start shopping for your new tub waste & overflow now!

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