What waste pipe 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 frequently asked questions when choosing drainage equipment. At first glance, a minor detail – the diameter of the drain pipe – can cause serious problems in practice: if you buy a trap or drain with the wrong connection diameter, you have no choice but to make another trip to the store, wait unnecessarily for the goods, and in the worst case, even tear up a newly tiled surface. That is why we are addressing this topic in great depth.

In this article, we will explain what the DN marking (Diameter Nominal – nominal diameter) means, why there are three most common sizes (DN32, DN40, DN50), where each is typically used, what the actual physical dimensions are, and how to proceed when you are unsure. Using practical examples, we will show typical mistakes that can be easily avoided.

What is the nominal diameter DN and why is it not the same as the actual pipe diameter?

DN (from the French Diamètre Nominal) is a standardized sizing system that defines the pipe diameter regardless of the material, wall thickness, or manufacturer. It is a reference number, not an exact internal or external diameter in millimeters – although it is fairly close in practice.

In the context of bathroom drainage equipment, it can be simplified as follows:

DN32 – internal diameter approx. 32 mm, external diameter of PVC pipe typically 40 mm
DN40 – internal diameter approx. 40 mm, external diameter of PVC pipe typically 50 mm
DN50 – internal diameter approx. 50 mm, external diameter of PVC pipe typically 58–63 mm (depends on the manufacturer and system)

Therefore, when purchasing, always work with the DN marking, not just with millimeter measurements measured with a caliper somewhere in the apartment – the external diameter of the pipe will tell you the pipe diameter, not the drain diameter to which the trap connects. It could also cause confusion.

DN32 – where is it typically used?

DN32 is the smallest commonly used diameter in bathroom drainage equipment. In a household, you will mostly find it in places where the water flow is relatively small and where space limitations under the fixture require a compact solution.

Typical uses of DN32

Sinks – the classic standard for sink drains. The vast majority of sink traps (bottle, elbow, built-in) have a DN32 outlet. The water flow when washing hands is low enough for this diameter to handle it without problems.
Bidets – also standard DN32.
Some older bathtubs – in buildings from the 60s to 80s, we occasionally encounter DN32 on bathtubs, but this is now more of an exception. During renovations, we always recommend upgrading to DN40.
Kitchen sink (sometimes) – some older sinks had DN32, modern ones typically use DN40.

Important practical note: never use DN32 for a shower tray or bathtub in a new build or major renovation. This diameter is simply not sufficient to drain a larger volume of water, and the result is standing water in the shower, which is not only unpleasant but can damage silicone seals and grouting over time.

DN40 – the golden standard for the bathroom

DN40 is today the most common diameter in modern bathrooms. If you have a relatively new building (post-2000) or are doing a renovation, you are very likely to encounter DN40.

Typical uses of DN40

Bathtubs – the absolute standard. Practically all modern bathtub traps, including automatic systems such as Automatic bathtub trap with overflow, chrome, are designed for DN40. A bathtub drain must handle the rapid emptying of the entire volume (150–300 liters), so DN40 is essential.
Shower trays with a bathtub – trays with a 52 mm drain hole (which is the typical external diameter for a DN40 drain) are standard on the market.
Floor drains in shower trays – for example, Floor drain with chrome grid 12×12 cm, DN 40, brass is a typical example. A floor drain with DN40 can handle the typical flow from a shower head (8–15 liters per minute) with sufficient reserve.
Modern kitchen sink – double sinks, large kitchen sinks with waste disposal units.
Washing machine (drain) – the washing machine outlet is usually connected to a DN40 trap.

DN50 – when do you really need it?

DN50 is the largest diameter of standard bathroom drainage equipment and its use is justified primarily in cases of higher flow or specific structural conditions.

Typical applications of DN50

WC – toilet bowl – WC waste is exclusively DN50 (or DN100 on the main drainage pipe, but the neck directly from the bowl to the waste pipe is DN50 or a reduction to the drain).
Floor drains with higher flow – longer drains (over 900 mm) or drains in commercial areas, gyms, sauna rooms, where water flows from multiple shower heads at once. In such cases, DN40 is insufficient and overflow is a risk.
Floor drains in wet technical rooms – boiler rooms, laundry rooms, cellars.
Barrier-free showers without a bathtub (walk-in) – in large shower zones or in the case of using a rain shower head with high flow (over 20 l/min), DN50 is more suitable.
Certain premium floor drains – for example, the long Flexible floor drain with an edge for a perforated grate and an adjustable wall flange, length 850 mm – for such lengths, the manufacturer specifies what drain it can handle, and DN50 may be directly specified in the technical data sheet.

It is important to understand that a larger diameter is not automatically better. If you have DN40 piping in the wall and you buy a drain or a drain with a DN50 outlet, you will have to deal with a reduction – and every reduction is a potential clogging point. Always check what you have in the wall or in the floor, and choose the device accordingly.

How to find out what diameter you have in the wall (or in the floor)?

This is the most common practical question during renovation. There are several reliable methods:

Method 1: Direct measurement of the existing pipe

If the waste pipe is accessible (e.g., when replacing a trap, when the old trap is removed), measure the outer diameter of the pipe with a caliper and compare:

Outer diameter ≈ 40 mm → pipe is DN32
Outer diameter ≈ 50 mm → pipe is DN40
Outer diameter ≈ 58–63 mm → pipe is DN50

Method 2: According to the device that drains into it

If you know what drains into the waste (sink, bathtub, shower), you can rely on the standards mentioned above. In an apartment block built after the year 2000, it is practically certain that bathtubs and shower trays have DN40.

Method 3: Building documentation

In a new build, the plumbing layout including dimensions should be indicated in the project documentation. This method is reliable, but documentation is not always available.

Method 4: Ask the manufacturer or supplier

The manufacturer of the fixture (bathtub, shower tray) has the required drain diameter specified in the technical data sheet. If you are buying a new bathtub, simply check what it requires.

Bathtub traps and DN diameter – what you need to know

Bathtub traps are an interesting case in terms of diameter, as they have two different connections: the drain of the bathtub itself (standard 52 mm outer diameter of the hole in the tub) and the outlet to the waste pipe.

The outlet of a bathtub trap into the wall (or into the floor) is standard DN40. This applies to almost all common bathtub traps, including Bathtub trap CLICK/CLACK with a removable outlet. This type of closing mechanism is very popular today precisely because the drain can be opened and closed with foot operation without bending – and it works reliably thanks to the DN40 sizing, which allows for quick draining of a full bathtub.

In older buildings (panel houses from the 60s–70s), you may encounter DN32 on the bathtub waste. In such a case, we strongly recommend upgrading the system to DN40 during renovation – it is not a dramatic job, but the result is significantly better drainage and a lower risk of clogging. Hair and soap residue clog DN32 much faster than DN40.

Floor drains and grates – diameter and flow in numbers

When choosing a floor drain or grate, DN is not just a technical parameter – it directly determines the maximum flow that the drainage can handle without overflowing. Let's look at real values:

DN40 floor drain: usually handles 20–28 liters per minute. A standard shower head consumes 8–12 l/min, so there is a large margin here.
DN50 floor drain: usually handles 40–50 liters per minute. Justified in the case of multiple shower heads or a rain shower with large-area spraying.
DN40 floor grate (750–900 mm): maximum flow depends on length and slope, but it ranges from 20–35 l/min.
DN50 floor grate: 40–65 l/min – suitable for long grates or commercial applications.

For a standard home shower tray, DN40 is fully sufficient. If you are dealing, for example, with a double-sided 750 mm floor drain DOUBLE9 FIT and GO, stainless steel, the manufacturer specifies an optimal drain diameter for this type – in this case, you will use DN50 as the recommended outlet, which makes technical sense for a 750 mm length and intensive shower flow. Always read the technical specifications of the specific product, not just estimate based on appearance.

The topic of choosing between a floor drain and a floor grate is discussed in more detail in the article Floor drain vs. floor grate: what is better for a shower tray? in this Knowledge Center.

Reductions between DN – when yes, when not?

Sometimes the existing installation is DN40, but you want to use a device with a DN50 drain, or vice versa. In such cases, a reduction (reduction neck, rubber transition-reduction or elbow) comes into play.

When a reduction is acceptable

Reduction from DN50 to DN40 (reducing the drain) is technically possible, but you should consider whether the DN40 pipe in the wall can actually handle the flow for which the device is designed.
Temporary solution while waiting for pipe system renovation.
Short reductions directly at the trap, if the rest of the piping is correctly dimensioned.

When you should avoid reductions

Reduction from DN32 to DN50 along the entire section – instead, renovate the piping system.
Reduction placed at the bottom of the siphonic trap (where dirt accumulates) – increases the risk of clogging.
Long sections with reduced diameter – slows down drainage and may create a siphon effect with suction of odors.

From practical experience, we can say: reductions work if they are properly installed and used in a short section. Problems arise when they become a permanent solution in a place where a proper piping system is needed. Then it is just postponing the problem – and in the bathroom, water drainage problems always arise at the least convenient moment.

Practical scenarios from renovation practice

So we don't just go by theory, here are a few typical situations we encounter in practice:

Scenario 1: Replacing a bathtub trap in a panel house from the 80s

The customer is replacing an old bathtub trap. They remove the old one, measure the waste neck – the outer diameter of the pipe is 40 mm, so DN32. A new modern trap (DN40) will not fit there. There are two solutions: either you get a trap with a DN32 outlet (still available), or you renovate the waste line to DN40. The second option is the correct long-term solution – the first is a compromise that will work, but you will encounter the same problem again during the next renovation.

Scenario 2: New barrier-free shower tray with walk-in solution

New construction, a 1200×900 mm shower tray without a bathtub, with a long floor drain along the wall. The architect designed a shower head with a flow rate of 18 l/min. Correct choice: floor drain with DN50 outlet, directly connected to a DN50 waste pipe in the floor. Using DN40 in this situation would be on the edge – when the shower head and handheld shower are used at the same time (which everyone does), the drain might not be sufficient and water could start to accumulate on the floor.

Scenario 3: Renovation of the bathroom, the customer wants a "larger drain" for better drainage

The customer is replacing a DN40 floor grate with a "better" DN50 one, thinking it will provide better drainage. Problem: the pipe in the floor is DN40. The result is a reduction, which in reality does not improve performance – the limiting factor is the pipe, not the grate. The real problem was elsewhere: the old trap was neglected and dirty. After cleaning and replacing it with a new DN40 grate, everything worked perfectly. Lesson: a larger DN with the same pipe behind the device does not mean better drainage.

Scenario 4: Buying a floor drain on an e-shop without verifying DN

The customer buys a drain with a DN50 outlet, but the floor has a DN40 system. The drain arrives, the installer discovers incompatibility. Solution: a reduction is searched for, a day and some extra euros are lost. This can be easily avoided – before purchasing, verify what DN you have in the floor. If you don't know, call the installer or ask when ordering.

Special situations: outlet diameter vs. overflow diameter

Bathtub traps have not only an outlet neck (lower part, where it connects to the waste pipe) but also an overflow neck (upper neck connected to the overflow hole in the tub). The diameters of these two necks can be different and should not be confused:

Outlet hole of the tub (lower part, at the bottom of the tub): standard outer diameter 52 mm – this is not DN marked, it refers to the diameter of the plug/trap flange
Overflow hole of the tub (upper part, overflow barrier): standard outer diameter of the flange 55–62 mm
Trap outlet to the waste pipe: DN40 (outer diameter of the pipe 50 mm)

Therefore, when purchasing a bathtub drain, always specify the manufacturer and model of the tub – or directly provide the dimensions of the openings in the tub to ensure compatibility. For more information on choosing the right bathtub drain, see the article How to choose the right drain: sink, bathtub or shower? in this Knowledge Center.

Summary: quick decision-making process

If you want to quickly verify which DN you need, follow these steps:

Find out which device you are draining (sink → DN32, bathtub/shower → DN40, WC → DN50).
Check which DN you have in the wall or floor (measure the outer diameter of the existing pipe).
Both numbers must match – the device and the piping must have the same DN (or the device can have a larger DN and you can solve the reduction, but ideally not).
In specific cases (long trough, large shower tray, multiple showers), consult with an installer before purchasing.

Most frequently asked questions (FAQ)

Can I connect a DN40 trap to a DN32 pipe?

Technically yes, using a rubber reducer or a reducing neck. In practice, however, this will limit the flow to the capacity of the DN32 pipe, which can cause slow drainage and frequent clogging in the case of a bathtub or shower. We recommend upgrading the waste line to DN40 – it is a one-time job that pays off in the long run.

Does the DN diameter also affect bad odors from the trap?

Indirectly yes. A too small diameter (DN32 on a device where DN40 should be) can lead to slower drainage, water stays longer, deposits form and they smell. At the same time, negative pressure in a poorly dimensioned pipe can "suck" the trap – push water out of the water seal and let waste gases into the bathroom. More in the article Smells from the trap or standing water: causes and solutions.

Is DN50 better than DN40 if I have the option to choose?

Not necessarily. A larger diameter brings a higher flow capacity, but if the piping is not dimensioned for it, it won't help. For a standard home shower tray, DN40 is optimal – it has sufficient capacity and is much more readily available in the range of traps, grates and troughs. DN50 is selected only where the flow is genuinely higher.

What does it mean when a trough or grate says "DN40/50"?

Some products offer drainage in two sizes – you can choose which neck to install according to the piping you have. Usually, the trap insert or drain neck is replaceable. Always check how a specific manufacturer solves it – sometimes it is about compatibility with reducing inserts, sometimes it is about genuinely different product variants.

Do the same rules apply for plastic and metal pipes?

From the perspective of DN marking, yes – DN is the nominal diameter independent of the material. In practice, however, plastic (PVC, PP) and copper or steel pipes may have slightly different actual outer diameters for the same DN, so it is always better to know the specific material and outer diameter of the pipe when purchasing seals and reducers, not just the DN number.

Where can I find products with a specific DN diameter on atria.sk?

In the category drainage technology, products are filtered and the description of each device includes the drainage diameter. If you are unsure, contact details and technical support are available directly on the site. When choosing a floor drain, we also recommend checking the relevant technical sheet – you will always find the exact drainage diameter and maximum flow rate in l/min there.

Conclusion

Selecting the correct DN diameter is not rocket science when you know what to look for. A simple rule: sink and bidet = DN32, bathtub and shower = DN40, WC and larger troughs = DN50. Always check what distribution you have in the wall or floor and adjust your purchase accordingly – not the other way around. Reducers are emergency solutions, not standard.

Whether you are replacing an old bathtub trap, solving a new shower tray with a floor trough, or planning a completely new bathroom – the DN diameter should be the first thing you find out, not the last. This will save you unnecessary costs, frustration, and in some cases even the demolition of freshly laid tiles.

For further technical details on installation, device selection and solutions to common problems, continue reading the other articles in this Knowledge Center – for example Installation of a floor trough step by step: slope, position, connection or Installation of a bathtub trap with an automatic or CLICK/CLACK system.

Do you have a question about this topic?

Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.