Standard Tub Drain Size and Guidelines

The standardization of measurements across the plumbing industry means that there is very little choice when it comes to different sizes of plumbing fixtures and fittings. In most scenarios, an excess of choice is regarded as a good thing, but in plumbing, having less choice is actually an advantage.

This makes it much easier to complete plumbing work because you don’t have to waste time taking measurements, and it is easier to source fittings from different manufacturers because they will all be of a standard size. You can find the standard tub drain size below. 

A standardized drain diameter for a bathtub drain is around one and a half inches. Non-standard drain diameters such as antique and vintage bathtubs will typically have slightly smaller drains measuring 1 ⅜ inch, but you can use a reducer washer to fit the 1.5-inch pipework.

Standard tub drain sizes
Illustration: © Homenish, 2023, Standard Bathtub Drain Sizes

Standardized Bathtub Drain Diameter

When it comes to the drain size on a bathtub, there is only one size that is considered standard across the board. The measurement for the diameter of a bathtub drain is 1 and a half inches.

The diameter is the measurement taken from one side to the other, cutting directly through the center point. This is helpful for builders and plumbers because when installing a new bathtub, they will know to use rough-in piping that has a diameter of 1.5 inches.

Even if the plumbers are not involved with the final fix install of the bathroom and don’t know what type of bathtub will be chosen, they can be sure that the drain will work in correspondence with the pipework due to the fact that there is a standard size.

Non-Standard Bathtub Drain Diameter

Older bathtubs, such as antique bathtubs and vintage bathtubs, will typically have slightly smaller drains measuring 1 ⅜ inch.

If you want to install a vintage clawfoot bathtub in your new renovation, then this can be achieved by using a reducer washer, which will mean that the smaller drain hole can still be attached to the 1.5-inch pipework.

Fortunately, it is quite rare to come across a drain that does not measure the usual 1.5 inches in diameter.

This is the standard bathtub drain size, no matter how big the bathtub is, no matter who manufactures the bathtub, and no matter what country you are in. The vast majority of all bathtub drains, wherever you may go in the world, will have a diameter measurement of 1.5 inches.

Choosing a Bathtub Drain

Choosing a Bathtub Drain

Most bathtubs come with drains, so when you buy a new tub, this isn’t something you will typically need to think about. Sourcing a new tub drain is more common in instances where your old drain has broken or become damaged in some way, and it is then that you’ll be asking yourself what sort of drain you need.

Since drains are of a standard size, you can buy pretty much any bathtub drain, confident in the knowledge that it will be compatible with your tub. Since all of the sizes are exactly the same, you don’t need to confuse yourself with dimensions; instead, just focus on the look and functions of the drain.

Most drains will be silver in color, finished in stainless steel since this has for several decades been widely accepted as a classic, modern choice.

However, you may have fittings in your bathroom that are a different color, for example, gold, brass, copper, or matte black, and in this instance, you will probably want to find a drain for the bathtub to match in order to create a seamless and coordinated look.

Bathtub Drain Length

If you are curious as to the length of a bathtub drain, maybe to ensure you are buying one that is going to fit in with your plumbing, then it is probably not the drain you are actually thinking of. Bathtub drains are very shallow and do not have a length measurement.

Even if they did, the length measurement would be irrelevant because all bathtub drains are the same size and will fit interchangeably with any bathtub.

However, the P-trap is what connects to a drain beneath the surface of the tub, and these can have different lengths and sizes. The P-trap has a curving shape, which makes the size difficult to gauge; however, if you are concerned about whether it will be compatible with your drain, then do not worry because all P-traps have a standard opening size of 1.5 inches so that they will work with any bathtub drain.

The purpose of a P-trap is to carry wastewater out of your drain and down to the sewer or septic tank, and so it has a very important function.

The curve in this piece of plumbing is U-shaped, and it is designed to hold water inside the curve using gravity, thus preventing anything unwelcome, like odors or vermin, from making their way into your bathroom via the drain pipe.

The fact that the P-trap has the job of trapping water is what gives it its name.

If you need to replace your P-trap, then you should measure the one you already have and find one that is the same size, as this will ensure it fits in with your plumbing work.

If you have any doubts, call a plumber for help.

Other Drain Sizes


The drain size in a shower is much bigger than that in a bathtub. In fact, shower drains are more than twice the size of bathtub drains at 3.25 inches.

The opening of the P-trap in a shower is just 2 inches; however, the shower drain is designed in such a way that these components are easily compatible. Basin drains, unsurprisingly, are smaller than bathtub drains.

Basin and Bathroom Sink

The size of basin drains is standardized at 1.25 inches; however, there does seem to be some variation in this between different sizes of basins and different manufacturers.

Some larger basins and bathroom sink drain pipe sizes can measure 2 inches in diameter.

If you need to replace your basin drain, always be sure to check the size you need beforehand by measuring the diameter across your current basin drain, as this is the one piece of the bathroom where you are most likely to find a non-standardized size of the drain.