bathtub drain

Bathtub Drain Sizes

If you’re installing a new bathtub, you might be worried about making sure that you have the right piping. But you don’t need to be.

The drain setup might seem complicated, but it’s fairly straightforward. There aren’t many different components, and all drains come with a standard size of 1.5 inches.

These are some of the most important things that you should know about the size and setup of your bathroom drain.

What Is The Standard Bathroom Drain Size?

With a bathtub, the drain will be 1.5 inches in diameter. The drains for sinks can vary slightly, but the most common measurement for those is 1.25 inches.

You might also find sinks with a drain diameter of 1.625 inches.

Thanks to the standardization, any bathtub drain can fit any standard bathtub between 14 and 26 inches in depth.

The size of the drain is standardized because it needs to fit to the P-trap, which is the next component of the drain. This opening also has a standard size of 1.5 inches.

P-traps don’t have a standardized length, though, so it’s important to get the right one to fit your plumbing.

Are There Different Size Bathtub Drains?

Standard bathtub drains remain the same no matter what tub you use, what manufacturer you use, or what country you’re in. This makes it a lot easier to put together the plumbing components, as well as to replace any worn-out pieces.

It’s possible that some very old or unconventional tubs might not have a standard-sized drain. But this is the exception rather than the rule, and it would be a big inconvenience.

One important note is that shower stalls have differently sized drains. These are more than twice as big, with a total diameter of 3.25 inches.

They are also wider than the opening of a shower drain’s P-trap, which is just 2 inches in diameter.

How Is Bathtub Drain Diameter Measured?

The diameter of the drain is measured by taking the size of the opening at its widest point. From one edge of the hole to the other is a length of 1.5 inches.

How Long Is The Bathtub Drain?

When you ask about the length of a bathtub drain, usually you’re thinking about the length of the P-trap. This piece connects to the drain opening and has a standard diameter of 1.5 inches.

However, it does not have a standard length.

P-traps are curved shapes, and it is difficult to alter their dimensions. Because of this, it’s important to know what size you’re looking for before you get started.

It is hard to measure the space for a P-trap due to the curved bend. You need to measure each length surrounding the bend.

The drain pipe itself will be fitted to the P-trap, at which point the water will be carried through the rest of your plumbing.

How Do You Choose A Bathtub Drain?

bathtub drains

Since bathtub drains have a standard sizing, you don’t need to worry much about finding the right opening for your tub. But you might need to worry about how components like the drain pipe and P-trap will fit with the rest of your plumbing.

With the dimensions set aside, the next consideration is the style of the drain. You’ll also want to consider purchasing an overflow drain to keep your bathtub from becoming too full.

The building codes in your city or state might govern your choices of bathtub drain. There may be guidelines in place for the size of components like the P-trap and the drain pipe.

There may also be requirements for your overflow drain.


Finding the right bathtub drain size shouldn’t be a problem. All standard tubs are built with a 1.5 inch drain.

If you find an antique tub, there might be a slightly different measurement, but this is an unusual circumstance.

The drains are standardized so that they can fit easily with the attached P-trap. Drains for your sink have a different size.

And if you want to install a shower stall drain, that will be more than twice as wide as a bathtub drain.