What drain diameter do I need: DN32, DN40 or DN50?

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What drain diameter do I need: DN32, DN40 or DN50?

One of the most commonly underestimated decisions when renovating a bathroom is choosing the correct diameter of the drain pipe. Most people only address it when they have a trap or a drain in their hand and realize that the waste pipe under the floor has a different diameter than expected. The result? Another trip to the store, unnecessary costs for cutting PVC pipe or – in the worst case – a built-in mistake that will manifest as water seepage after years.

In this article, we will thoroughly examine what the designations DN32, DN40 and DN50 actually mean, where each diameter is used, how it affects the choice of trap, drain or floor drain, and what happens if you make a mistake. We will rely on specific examples from everyday installation practice, because it is precisely in the details that most problems are hidden.

What does "DN" mean and why it is not just a number on paper

The abbreviation DN comes from the French "Diamètre Nominal" – i.e. nominal (nominal) diameter. It is a standardized European designation used for pipes, fittings and entire drainage systems. An important point to understand right from the start: DN is not the outer diameter, it is not the inner diameter – it is a standardized numerical value that corresponds to the nominal opening. In practice, the actual inner diameter is usually very close to the DN number, but not always exactly the same.

In everyday bathroom sanitation, you will most often encounter these three values:

DN32 – nominal diameter 32 mm, pipe inner diameter approx. 29–32 mm
DN40 – nominal diameter 40 mm, pipe inner diameter approx. 37–40 mm
DN50 – nominal diameter 50 mm, pipe inner diameter approx. 47–50 mm

When you read on the box in the store "trap DN40", it means that the drain opening of the trap is dimensioned for a DN40 pipe and you will use a pipe or flexible hose with an outer diameter corresponding to this standard for connection. Any other pipe diameter will require a reducer, which is an additional installation complication.

DN32 – when is the smallest diameter sufficient

DN32 is historically the most commonly used diameter for sink traps. If you have older housing from the 70s–90s, it is likely that the waste under the sink is exactly DN32. This diameter is dimensioned for the discharge of cold water during normal hand washing, tooth brushing or washing dishes in small quantities.

The flow capacity of a DN32 pipe is approximately 0.5–0.8 liters per second at a standard slope. This is sufficient for a sink – standard taps on sinks have a flow of 6–10 liters per minute, which is only 0.1–0.17 l/s. So even with a fully open tap, DN32 will easily drain the water without the risk of overflow.

Where DN32 is not sufficient:

Shower cubicles – thermostatic valves and rain shower heads have a flow of 15–20 l/min or more
Baths – draining a bath with a volume of 150–180 liters must be done in a reasonable time
Floor drains – collect water from a larger area
Kitchen sink with a waste disposal unit – a large amount of water flows at once

In practice, I have repeatedly seen situations where a flat owner bought a cheap shower cubicle with a DN32 trap and after a few months complained that when taking a long shower, water would stand up to the ankles in the shower. The cause was simple: the flow of the shower was higher than the capacity of the drain. No clogging, just an undersized diameter.

DN40 – the golden mean for most bathroom applications

DN40 is currently the standard diameter for shower cubicle traps, bath traps and most floor drains. If you are renovating a bathroom and you don't know what waste you have and cannot measure it (e.g. it is embedded in concrete), the safe choice is DN40 in 80% of cases.

The flow capacity of DN40 is approximately 1.0–1.5 liters per second at a normal slope of 1–2%. This comfortably handles:

Thermostatic shower valves (flow 12–18 l/min)
Rain shower head 30 × 30 cm (flow up to 20 l/min at normal pressure)
Bath – a 150-liter bath can be drained in 2–3 minutes
Floor drain in a shower cubicle of approx. 1.5 × 1.5 m

A typical example of a product dimensioned for DN40 is Floor drain with chrome grid 12x12cm, DN 40, brass. This drain is suitable for shower cubicles of standard size and its DN40 drain covers the flow of a standard shower with a reserve.

For bath traps, DN40 is also standard. A bath trap – whether a classic Bath trap with automatic overflow, chrome or a more modern Bath trap CLICK/CLACK, with removable overflow – works with a DN40 drain opening, which is the optimal choice for a bath. More on the choice between automatic and CLICK/CLACK system can be found in the article Installation of a bath trap with automatic or CLICK/CLACK system in this Knowledge Center.

DN50 – when a larger diameter is necessary

DN50 is the largest commonly used diameter in the bathroom and is the standard for long floor drains, large walk-in showers, and situations where multiple fixtures drain into a single point. The flow capacity of DN50 reaches approximately 1.8–2.5 l/s under normal slope conditions.

Where DN50 is justified or even necessary:

Floor drains longer than 700–800 mm – the longer the drain, the greater the volume of water that must drain per second. An 850 mm drain such as Flexible – Floor drain with edge for perforated grate and adjustable wall flange, length 850mm is at the borderline where manufacturers usually install a DN50 drain.
Walk-in showers with multiple heads – a combination of ceiling head + handheld shower + side sprays can reach a flow of 40–60 l/min
Double floor drains – a product like Floor drain double-sided, 750mm, DOUBLE9 FIT and GO, stainless steel has larger catchment areas from both sides, which requires an appropriate drain
Shared drain for bathtub + shower on a single pipe section
Floor drain in a larger wet area (e.g. wellness, sauna with a bath, pool area)

In practice, I often encounter situations where people install a beautiful long drain in a large walk-in shower, but connect it to DN40, because "it has always been done that way". The result is that during a shower with two people and fully open fixtures, the water cannot drain fast enough, and they always get the feeling that the drain is clogged. In reality, the drain is clean – only the pipe cannot drain the water. Repairing a built-in drain is an expensive job.

Where is the waste located and why it complicates the selection

Before you choose any trap, overflow or drain, you must know the diameter of the existing waste pipe in the wall or floor. This is not always easy to determine, so here are some practical tips:

1. Measure the outer diameter of the existing pipe. Common PVC drainage pipes have the following outer diameters:

DN32 → outer diameter ~40 mm
DN40 → outer diameter ~50 mm
DN50 → outer diameter ~63 mm

If you have access to the pipe and can measure the outer diameter with a caliper, you are in the best position.

2. If the pipe is embedded in concrete and you only see the opening, measure its inner diameter. Values close to 32, 40 or 50 mm will indicate the DN.

3. If you cannot measure at all – for a shower overflow in new buildings (since about 2000) it is almost always DN50 or DN40. For a sink DN32 or DN40. For a bathtub most often DN40.

4. Distinguish the material of the pipe. Old cast iron pipes may have different dimensions than modern PVC. If you encounter a cast iron pipe, I recommend consulting a plumber, as reductions for cast iron drainage are special.

How the drain diameter relates to specific products

Bathtub traps and their drains

Bathtub traps almost exclusively use a DN40 drain. This is due to the fact that the bathtub has a standard drain hole of 52 mm in diameter at the bottom, and the trap is mounted into it via a gasket. The trap's drain neck leads to the wall or floor and connects to a DN40 pipe.

However, it is important to distinguish between the bathtub's drain hole (DN52 or 52 mm) and the trap's drain neck (DN40). These are two different dimensions at two different locations. If you are replacing a trap and see the hole in the bottom of the bathtub, do not measure it – it is always ~52 mm. Measure the pipe to which the trap connects.

Modern bathtub traps such as the CLICK/CLACK system have the advantage over traditional automatic traps in that the drain can be cleaned without disassembly. Both versions, however, use the same DN40 drain neck. More on this topic can be found in the article How to choose the right trap: sink, bathtub or shower?

Floor drains and their dependence on length

With floor drains, there is a direct relationship: the longer the drain, the larger the drain you need. Manufacturers either fix the DN (e.g. all drains over 800 mm have DN50), or offer drains with an optional drain module.

For orientation:

Drain length	Recommended DN drain	Reason
up to 500 mm	DN40	small catchment area, low flow
500–800 mm	DN40 or DN50	depends on the shower flow, DN50 is recommended
over 800 mm	DN50	high volume, requires a larger drain
over 1000 mm (walk-in)	DN50, possibly 2× DN50	large area + multiple fixtures

The detailed installation of gutters, including proper connection to the drain, is covered in the article Installing a floor drain step by step: slope, position, connection, where you will also find information about the adjustable flange – a topic covered in a separate article Adjustable flange for a floor drain: when do I need it and how does it work?.

Basin traps – DN32 vs. DN40

This is one of the most common sources of confusion. Basin traps are available in both variants – DN32 and DN40. How to determine which one you need?

Practical rule: if you have a building constructed before the year 2000, it is likely DN32. If you are in a new building or a house with a completely renovated drainage system, it is likely DN40. If you are unsure, measure the outer diameter of the visible part of the drain pipe under the sink. Alternatively, unscrew the old trap and look at the neck that remains in the wall – the inner diameter of the neck will reveal the DN.

For DN32 basin traps, flexible hoses usually have an outer diameter of ~40 mm. For DN40, it is ~50 mm. Do not confuse these values with the nominal diameter – it is the outer diameter of the flexible hose, not the DN marking.

Reductions: help or problem?

If the diameter in the wall is different from what your trap or drain requires, there are reducing collars and connectors. A reduction from DN50 → DN40 or DN40 → DN32 is a standard item in any hardware store. However, reductions come with several potential problems:

Reduced flow – if you reduce a DN50 drain to a DN40 pipe, you lose part of the capacity you intended to gain by using a larger drain
Accumulation of dirt – at the reduction site (the step in the inner diameter), hair, soap deposits, and dirt accumulate, which speeds up clogging
Sealing – each reduction is a potential leak point if not properly sealed and secured
Depth of installation – a reduction takes up space, which can be a problem with shallow drains

My recommendation: always prefer, if at all possible, a product with a diameter that exactly matches the existing pipe. Use a reduction only when changing the pipe is not feasible (e.g., it is embedded in concrete). In such a case, the reduction will of course work, but be aware of its limitations.

Special case: what to do with DN75 or DN100 pipes?

In some older buildings, especially in villas or single-family homes from the beginning of the 20th century, you may encounter drainage pipes DN75 or even DN100 directly in the bathroom. These are vertical stacks (risers) or horizontal drainage pipes dimensioned for the entire apartment unit.

In this case, you never connect a trap directly to DN100 – that would make no sense. There is always a reducing piece or a "Y" branch, where the DN100 riser has a side branch with a smaller DN40 or DN50 for the bathroom. If you want to connect differently during renovation, always consult with a plumber – this is a main drain, and a layperson should not interfere without the knowledge of the housing company or building manager.

Slope of the pipe and its influence on DN selection

The pipe diameter itself does not determine the flow performance on its own – equally important is the slope of the horizontal drainage pipe. For drainage pipes in the bathroom, a minimum recommended slope of 1–2 % (10–20 mm per meter) applies. With a smaller slope, water does not move fast enough, solid residues settle, and the pipe clogs much sooner.

If you have spatially limited conditions (low space between the floor and the ceiling below, long distance to the riser), it may happen that you physically cannot achieve the correct slope for DN40. In such a case, either increase the diameter to DN50 (a larger diameter with the same slope = higher flow, so you can tolerate a slight lack of slope), or consult with a plumber about the pipe routing.

Practical rule: if the distance from the drain to the riser exceeds 3 meters, consider DN50 even for a standard-sized shower, because a long horizontal pipe with decreasing slope really deteriorates the drainage performance.

Most common mistakes when selecting DN – examples from practice

Example 1 – new bathtub on old drainage: A customer bought a modern bathtub and a new trap. The old trap was DN40, and the new trap was also DN40, but the trap's outlet had a different length and direction, so he connected it to a DN40 flexible hose, but the hose had an inner suspension into the pipe that was 2 cm shorter. Result: a micro-leak with every filling. Solution? Measure correctly and buy a flexible hose of the correct length, not just the correct DN.

Example 2 – long drain on DN40: Bathroom renovation, the owner chose a 1000 mm drain for a large shower area. The plumber connected it to the existing DN40 drainage. The drain worked, but when two people were showering at the same time, using a 40 cm rain shower head, the water in the drain rose to a height of 3–4 cm and drained slowly. Problem: the drain was dimensioned for DN50, DN40 was not enough. The solution would have been to re-route the pipe, which required cutting into the floor and additional costs several times higher than the cost of the drain itself.

Example 3 – reduction as a temporary solution: In an old panel apartment bathroom, the drain was DN32 and the customer wanted a modern shower with a DN40 inlet. A reduction DN40 → DN32 was installed. It worked, but after two years the customer had constant problems with slow draining water. Cause: a combination of underdimensioned DN32 and the reduction, where hair would get caught. More about such situations can be found in the article Smelly trap or standing water: causes and solutions and also in How to clean a clogged trap without chemicals.

Material of the trap and its relation to diameter – connections

The choice of material (stainless steel, brass, plastic) has a smaller impact on the drainage behavior in terms of flow than the diameter, but it is indirectly related to how the pipe gets clogged and how easily it can be cleaned. Brass and stainless steel drains have a smoother inner surface than some plastics, which slightly improves the flow and slows down sedimentation. We will elaborate on this topic in the article Stainless steel, brass or plastic: what to choose for bathroom drainage equipment?

For comparison – if you have a DN40 brass trap vs. a DN40 plastic trap, the flow difference is minimal and practically negligible. More important is the internal shape of the water seal and the transition through the elbow. Cheaper plastic traps sometimes have sharper internal transitions, which slightly increase flow resistance. With DN50, this difference increases a bit more.

Common questions (FAQ)

Can I connect a DN50 trap to a DN40 pipe?

Yes, technically it is possible using a DN50/DN40 reducer. Such a reducer can be bought in any sewer pipe assortment. However, you must consider that the actual flow will be limited by the DN40 pipe, not the DN50 trap. If you bought a large drain precisely for higher capacity, the reducer partially undermines this purpose. I recommend connecting to DN50 whenever it is technically feasible.

How can I find out what drain I have in the wall if it is built-in?

The most reliable way is to look into the distribution shaft or technical room where the vertical risers are visible. At the branch-off point for your bathroom, you will see a Y-piece with a side branch – its visible neck will reveal the DN. If this is not possible, measure the outer diameter of the existing pipe where it is accessible (e.g. under the sink). DN32 has an outer diameter of ~40 mm, DN40 ~50 mm, DN50 ~63 mm. Alternatively, consult a plumber who knows the typical installation practice of the era your building was constructed in.

Is DN40 enough for a walk-in shower 1.2 × 1.2 m?

It depends on the shower fittings. If you have one standard handheld shower with a flow rate of up to 12 l/min, DN40 is sufficient. If, however, you plan to use a thermostatic valve with a 40 cm rain shower head (flow rate 18–25 l/min) plus a handheld shower, I recommend DN50. It is always better to dimension with a reserve – redoing embedded piping later costs several times more than the price difference between DN40 and DN50 products.

Does it make sense to buy a DN50 trap for a sink?

No, it does not make sense technically or practically for a sink. A sink drain is always DN32 or DN40 – DN50 would not connect anywhere. In addition, a sink never produces such a flow that DN40 would be insufficient. Focus on the correct DN according to your existing piping and not on the idea that bigger is always better.

What happens if you install a DN32 trap on a DN40 pipe?

The trap physically does not fit into the pipe without a reducer – the neck is 8 mm narrower and would loosely fall to the bottom without sealing. This would cause a leak. A DN32 → DN40 reducer is technically feasible when going from smaller to larger, but it reduces the capacity to DN32. This is not suitable for a shower inlet. For a sink, it might work, but again – better to buy a product with the correct DN.

Do the same DN sizes apply to floor drains and inlets?

Yes, DN designation is standardized and applies to all drainage devices the same – traps, inlets, drains, fittings. DN40 on a trap connects to DN40 pipe the same way as DN40 inlet. The difference may be only in the type of connection (sliding, threaded, rubber gasket), not in the diameter itself. Therefore, if you have a DN50 branch in the floor, buy a drain with a DN50 outlet and connect them directly without reducers.

Conclusion: how to choose correctly then?

Choosing the correct drain diameter is not a science, but it requires knowing three key pieces of information before you go to the store or order a product online:

What diameter does the existing drain pipe have (measure it, don't guess)
What type of device you are supplying (sink, bathtub, shower, drain) and what is its flow rate
What is the length of the route to the riser and what slope is realistically achievable

If you meet these three conditions, the choice is usually clear. For a sink, go for DN32 or DN40 according to the existing pipe. For a bathtub, DN40 is standard. For shower trays up to 80 cm and standard fittings, DN40 is sufficient, for larger areas and more powerful fittings, always DN50. For floor drains over 800 mm, calculate with DN50 as a minimum.

And if you encounter a situation where you are unsure – in the entire category of drainage equipment, products have clearly stated DN directly in the name or parameters, so you can make the correct choice without having to guess anything.

Do you have a question about this topic?

Can't decide or are dealing with a specific situation in your home? Write to us – we are happy to help.