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Drain Trap Sizes for Bathtubs

Steve Smith

If you install a new bathtub, it is likely you must install a new trap as well. Only a properly sized trap will work with your bathtub. So, it is important to know the proper size for the p-trap. It is similar to other sink p-traps used in the bathroom and kitchen.

Bathtub P-trap Diameter

Tub drains connect to a p-trap.

Bathtubs have the exact same drain trap as the kitchen sink and bathroom sink. The drain trap is 1½ inches in diameter. It is sized to handle the water volume from the bathtub with ease. The standard pipe size makes it easy to install the right p-trap on your tub and faucet sinks. This trap connects to the bathtub drain and the home's drain line. It is installed right between the two to prevent odors and keep water from backing up through the drain.

Bathtub Drain Sizes

A bathtub drain is also 1½ inches in diameter. This drain hole size matches the p-trap pipe size, so reducing bushings are not required to connect the two pipes. Usually, a drain tail pipe slides into the p-trap and is secured with a lock nut and compression washer. Both drain tail pipes and p-traps are made of PVC pipe because it is durable, corrosion resistant and cost effective.

P-trap Length

The length of the P-trap is not a standard size, but it is hard to change this dimension. It conforms to the length of the two p-trap fittings from which the p-trap is constructed. Usually, p-traps are made from two curved fittings placed one on top of the other. Each fitting is no longer than six inches in most cases. However, the curved shape makes them difficult to measure. The drain pipe fits inside of one of the longer ends of one of the fittings. The purpose is to suspend water inside the pipe fittings using gravity to block gas and odors.

Other Measurements

There are other dimensions and measurements to be aware of when installing a p-trap. The first one is the length from the tub drain. This measurement varies and is usually dictated by space constraints beneath the tub drain, in the flooring joists. Usually, the trap is placed as close to the tub as possible. The other is the drain run to the main home drain lines. This changes with the home's plumbing layout and is only a concern when finding the proper sized pipes to connect the p-trap to your home drain.