What HT pipe diameters do I need for the toilet, sink, and shower

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What pipe diameters do I need for the toilet, sink, and shower

When you start a bathroom renovation or build from scratch, one of the first practical questions you’ll ask yourself is: what pipe diameter do I actually need? It may seem trivial, but in practice, I repeatedly see the same problem – either people buy pipes that are too small (because “the water will still drain”), or out of excessive caution, they buy unnecessarily large diameters, which complicate the entire layout and increase the cost of the project. Then there are cases where the diameter changes several times along the route without logic, leading to clogging, noise issues, and later malfunctions.

This article will give you specific numbers for each type of appliance, explain the logic behind those numbers, and show what happens if you neglect them. We will focus on the internal HT system layout – that is, the piping inside the building, usually from the appliance to the vent pipe or septic tank.

Why the diameter matters more than you think

HT piping (an abbreviation from the German Haus-Technik, meaning home technology) is designed for gravity drainage. This means the water does not drain under pressure – it drains by its own weight, along an inclined plane, with the correct slope. The pipe diameter directly affects several parameters:

Flow capacity – how many liters per minute the pipe can carry without filling up and causing problems with the trap.
Self-cleaning effect – waste water should carry solid waste with it. This only happens with the correct ratio of diameter, slope, and flow. A pipe that is too large with low flow will cause the solid matter to remain at the bottom while the water drains past it – sedimentation occurs.
Noise – a larger diameter with low flow creates turbulent air bubbles inside the pipe, which manifest as gurgling and suction effects that damage water seals in traps.
Pressure conditions – when several appliances are connected to a single vertical stack, each connection must be sized so that the back pressure from one appliance’s discharge does not affect the others.

These are not theoretical considerations. I have seen cases where a toilet was connected to DN50 (50 mm) instead of the correct DN100/110, and the trap in the adjacent shower regularly “gurgled” after each flush – because the air had nowhere to escape and was looking for a path through the nearest water seal. Such errors are hard to find and even harder to fix once everything is built in.

Toilet: always DN 100 or DN 110 – no compromises

The toilet is by far the most demanding appliance in the entire drainage system of the bathroom. With one flush (a cistern toilet, tank volume 6–9 liters), a large volume of water drains suddenly along with waste, paper, and solid matter. This requires a minimum clear diameter of 100 mm, which in the HT system corresponds to a pipe with an outer diameter of 110 mm.

Why exactly 110 mm? In the HT nomenclature, the pipe diameter is marked by the outer diameter. A DN 110 pipe has a wall thickness of 3.2 mm, giving an internal (clear) diameter of approximately 103 mm. This is just above the minimum of 100 mm required by European standards (STN EN 12056-2) for toilets with direct connection.

In practice, I encounter two common mistakes:

Connecting the toilet to DN 75 – sometimes this is done due to lack of space in the floor, or when an old apartment building has DN 75 piping and someone tries to “adapt.” Result: frequent clogs, weak drainage, need to flush multiple times.
Connecting the toilet to DN 125 or 160 – on the contrary, sometimes people think that a larger diameter = better drainage. It is not. With an overly large diameter, the speed of the draining water decreases so much that the solid matter remains on the walls and bottom of the pipe. Hard deposits form, which are the seeds of repeated clogs.

Another important thing regarding the toilet: the minimum length of the connecting pipe from the toilet to the vertical stack or vent should be as short as possible and with a slope of 1–3%. Longer horizontal sections without sufficient slope are prone to sedimentation with DN 110 and small water volumes. If you need to change the direction of the route, use the correct HT elbow – depending on the situation, a 45° or 87° elbow.

Sink: DN 40 or DN 50?

This situation is a bit more complicated, as practice and standards sometimes differ slightly in conditions. A sink produces a relatively small flow – a standard sink trap has a DN 32 or DN 40 outlet. The STN EN 12056-2 standard assigns a drainage unit (DU) of 0.3 l/s to a sink, which theoretically could be handled by a DN 40 pipe.

In practice, we distinguish two situations:

A single sink connected directly to the vent: DN 40 is sufficient. The advantage is simple installation and less space required in the wall.
One or more sinks, or a sink together with a bidet or washing machine: We recommend DN 50. It allows the connection of multiple appliances without the risk of pressure fluctuations and is easier to clean in case of a clog.

An important practical detail: if you have a sink with a DN 32 trap, you must use a reduction before the HT pipe inlet. For this purpose, the HT reducer is used, which allows a smooth transition between diameters without turbulence and without losing the self-cleaning effect. Never do a reduction with an improvised hose or sealing tape – this is the seed of a future leak.

Regarding the slope of the sink connection: the standard specifies a minimum slope of 2 % (2 cm per meter of length) and a maximum of 4 % (4 cm per meter). For shorter sections (a sink near the wall, where the vertical stack is 60–80 cm away), this is not a problem. Problems arise when the sink is far from the vertical stack – in this case, you must carefully plan the height of the trap to ensure that everything fits into the floor or wall with the required slope, without unnecessary excavations.

Shower: DN 50 as standard, but watch out for the type of drain

The shower area is an interesting case. The water flow from a shower set (shower head, possibly side jets) can vary greatly – from 6 liters per minute for saving models to 20–25 liters per minute for rain showers or combined systems. For a standard apartment shower, DN 50 is sufficient.

But be careful: the decisive factor is not only the pipe diameter, but also the type of shower drain (floor trap). Standard round or square shower drains have a DN 50 outlet. Linear drain channels (long drains, so-called "line drains") also have DN 50, but some premium models for large showers may have DN 75 outlet – in this case, the correct HT reducer must be used or directly DN 75 pipe along the entire route to the stack.

Another practical point: the shower drain is typically located in the floor, where there is minimal space for pipe routing. In renovations of old apartments with thick-walled concrete slabs (e.g., panel buildings with slab construction), the space under the shower drain is often only 6–8 cm. This rules out a large-radius elbow and requires compact fittings. In such a case, it is clever to use an HT 87° elbow with a short branch directly under the drain, and then proceed to the horizontal pipe with the correct slope.

Bathtub: DN 50 as a minimum, ideally also DN 50

A bathtub typically has a volume of 150–200 liters and produces a more pronounced flow when draining than a shower. Even so, DN 50 is sufficient, because the bathtub outlet is not as concentrated as a WC – the water drains gradually, not in a sudden rush. The bathtub trap typically has a DN 40 or DN 50 outlet.

When combining a bathtub with a shower (a bathtub where you can also shower), it is reasonable to lead the drain directly to the stack and not to try to connect the bathtub to the same branch as the sink, if it would significantly extend the route or complicate the slope.

Washing machine and dishwasher: a special case

Although the washing machine and dishwasher are not part of the original question about the bathroom, in practice these appliances are often connected to the same system. The pumped discharge of a washing machine can reach up to 60–80 liters per minute in short peaks (the washing machine pump operates under pressure). This is a fundamental difference from gravity drainage. Therefore:

Drain of washing machine/dishwasher always at least DN 50.
If connected to a common branch, this branch should also be at least DN 50, ideally DN 75, if shared by multiple appliances.
The washing machine hose outlet must be connected to a siphon-equipped section (e.g., WC siphon or separate siphon) – direct connection without a water seal is prohibited (wastewater law, EN standards).

Stack: where everything meets – DN 100/110 as a basis

The stack is the vertical part of the drainage system into which all connections from individual floors are connected. For an apartment building or a family house with multiple toilets, the stack is always at least DN 100 (external diameter 110 mm in HT system). If the stack were smaller than the WC connection pipe, it would be a physical nonsense – the water flow would have nowhere to go and air pockets would destroy all siphons in the area.

For a family house with one bathroom, the stack can be DN 75, if the WC is not connected or if it is a house without a toilet on that floor – but this is an extremely rare and specific case. In normal practice: stack = DN 110.

For maintenance access to the stack and distribution, an HT cleaning piece is used – a fitting with a removable lid, which allows the insertion of a hose or auger in case of a blockage without the need to disassemble the pipe. This is an element that is often omitted during system design and later it is very difficult to find a suitable place for it.

Table of diameters for common bathroom fixtures

Fixture	Minimum DN	Recommended DN	Trap outlet type	Note
WC (toilet)	DN 100	DN 110	direct connection	Without exception, always DN 110 in HT
Sink	DN 40	DN 50	DN 32 or DN 40	Reduction if trap DN 32
Shower	DN 50	DN 50	DN 50	For line drain possibly DN 75
Bathtub	DN 40	DN 50	DN 40 or DN 50	Reduction if trap DN 40
Bidet	DN 32	DN 40	DN 32	Can be shared branch with sink
Washing machine / dishwasher	DN 50	DN 50	hose Ø 22 mm	Siphon-equipped section required, not direct connection
Stack	DN 100	DN 110	—	Always the same or larger than WC branch

How diameters change when connecting branches

Basic rule of hydraulics in a gravity drainage system: the diameter must not decrease towards the stack. The further away from the source (the device), the larger or the same diameter. Specifically:

If you have a sink DN 50 and connect a branch from a bidet DN 40 to it, the diameter after the junction must be at least DN 50.
If you have a shower DN 50 and a bathtub DN 50 and connect them before the stack, the common branch should be DN 75 – and if a WC is added here, it should be DN 110.
The diameter is never reduced in the direction of flow. Reduction is only acceptable in the opposite direction of flow (e.g., when feeding a side branch into a larger one).

In practice, these transitions are solved by a combination of fittings – branches with different throat diameters, reductions and elbows. It is important to know the assortment you are working with. For example, standard HT 45° or 87° branches are available in diameter combinations such as 110/50, 110/75, 75/50, which directly reflect common connection schemes.

What happens if you choose the wrong diameter – concrete examples from practice

To avoid it being just theory, here are real types of situations that I repeatedly encounter in customer projects:

Case 1 – WC on DN 75: A customer bought an older studio apartment in a panel building and wanted to move the WC 1.2 meters further from the original location. To save money, he bought DN 75 pipe (cheaper, lighter). After installation, the WC worked, but once every 2–3 weeks, a plunger or chemical cleaner had to be used. Cause: DN 75 does not have sufficient cross-section for the flush volume, and after each flush, solid particles remained on the wall. Solution: replacement of the pipe to DN 110, plus a cleaning tee for future service.

Case 2 – shower and washing machine on a common DN 50 without the correct branch: In a bathroom of a family house, the shower and washing machine were connected directly into a Y-tee, both on DN 50, leading to a DN 110 stack. Problem: during washing (the washing machine pump worked under pressure), water from the shower trap bubbled and the trap slowly emptied. Cause: the Y-tee did not have the correct geometry (the connection angle was 90° instead of 45°), which caused pressure surges. Solution: 45° branch and extension of the shower connection branch.

Case 3 – reduction in the direction of flow: During the renovation of a house, the previous "handyman" connected the WC (DN 110) via a reduction to DN 75 and continued to the DN 75 stack. Reason: the stack was originally DN 75 for the toilet on the upper floor and he did not want to change it. Result: the WC did not work properly for one year without clogging. The stack had to be replaced with DN 110 for the entire height.

Practical tips for planning and purchasing materials

Based on the above, I can summarize practical recommendations that will save you time, money and nerves:

Always start from the stack and plan towards the devices. The stack determines the maximum diameter of the system, from which all branches are derived. If the stack does not exist and you are designing it, DN 110 is the correct choice in 99 % of cases for a bathroom with a WC.
Draw a schematic before purchasing. Mark each device, measure distances and height differences, calculate slopes. Only then buy pipes and fittings.
Purchase pipes 10–15 % longer than your measurement. Losses from cuts, measurement errors – all of this eats into your stock. HT pipes are sold in standard lengths of 0.5 m, 1 m, 2 m and 3 m, which allows you to combine them without significant waste.
Do not forget about cleaning tees. Every longer horizontal branch (over 5 meters) and every place where the route turns into the horizontal before the stack should have a HT cleaning tee. Otherwise, during a clog, you will be cutting into the wall.
Do not use 87° elbows where 45° would suffice. Every sharp turn slows down the flow and increases the risk of deposits. Where the route allows it, use gradual turning via two 45° elbows or a curved elbow.
Always check tightness before covering during installation. Fill the system with water, or use a test cap HT plug. A leak behind drywall or in a shaft is a nightmare.

Pipe slope: an inseparable partner of the diameter

We are talking about diameters, but it is necessary to at least stop at the slope – because diameter and slope form an inseparable pair. The correct slope for HT pipe according to the standard STN EN 12056-2 is:

For DN 50 to DN 75: recommended slope 2 % (20 mm per 1 meter), minimum 1 %, maximum 4 %.
For DN 100 and above: recommended slope 1–2 %, minimum 0.5 % for long sections.

Why the maximum slope? If the slope exceeds 4 %, water drains too quickly and solid particles remain behind – the same problem as with an oversized diameter. Gravity is effective only within certain limits. More on this topic can be found in the article HT Drainage Pipe Slope: What Angle is Correct and Why It Matters.

Consequence for planning the diameter: if you know that you have little space for slope (e.g., a lower floor ceiling directly under the bathroom floor), you can increase the pipe diameter, which allows you to have a milder slope for the same flow capacity. And vice versa – with a steeper slope, you can go down in diameter, but only within the limits of the standard.

Noise in HT pipes and how it relates to diameter

This is a topic that customers address more and more, especially in modern apartments with open layouts, where the stack is in the living room or bedroom. Standard PVC HT pipe is not sound-insulated. When water flows through it (especially with a WC), the sound spreads through the pipe and through the mounting clamps into the wall.

Noise gets worse with:

Too large a diameter (air in the pipe vibrates).
Too steep a slope (water falls and hits the pipe wall).
Sharper turns (turbulence).
Metal fastening without damping washer.

The solution is either to use special noise-reducing HT pipe (available on the market, with multi-layer walls or mineral filling), or to apply thorough sound insulation with mounting foam and wrap the pipes with mineral wool in problematic areas. The diameter alone will not completely eliminate noise, but its correct choice will minimize turbulent flow behavior.

Summary rules – quick reference

Before the FAQ section, let's summarize the most important points in a simple overview that you can print out for planning:

Toilet → DN 110 (external diameter of HT pipe), always, without exception.
Basin → DN 50 (or DN 40 if it is isolated and has a short run).
Shower → DN 50 (or DN 75 for large line drain outlets).
Bathtub → DN 50 (reduction to DN 40 at the trap).
Vertical stack → DN 110 if the system includes a toilet, otherwise DN 75.
Branch serving multiple fixtures → always a larger or equal diameter than the largest inlet.
The diameter must never decrease toward the vertical stack.
2 % slope is the golden standard for DN 50 and DN 110.

Frequently asked questions (FAQ)

Can I connect a toilet to a DN 75 pipe if the old house has a DN 75 vertical stack?

No, this is done and it is done incorrectly. The standard clearly prescribes a minimum of DN 100 (HT DN 110) for a toilet with direct connection. If the vertical stack is DN 75, you must replace it at least from the floor with the toilet to the main stack, or completely. A simple "adaptation" to DN 75 leads to chronic clogs, bad odors, and loss of warranty on the fixture. This situation is resolved by replacing the vertical stack – not by reducing the diameter of the toilet branch.

Can I connect a basin and a shower to a shared DN 50 branch before the vertical stack?

Yes, this is a common and correct practice, provided the branches are connected using a proper branch fitting (45° angle or sloped inlet) and the diameter after the connection continues with DN 50 or larger. Make sure the fitting is a proper branch, not an improvised T-fitting with a right angle – the latter would cause turbulence and bulging. A correct connection of multiple fixtures to one branch is also explained in the article How to properly connect HT pipes, elbows and branch fittings without leaks.

Do I have to use exactly HT DN 110 or is any other DN 100 pipe sufficient?

For internal waste piping (from the toilet to the vertical stack inside the building), the HT system with an external diameter of 110 mm is specifically designed. The KG system (for external underground piping) has different dimensions and materials. Mixing systems is not advisable – the sockets, rubber seals, and fittings are not mutually compatible. For more information on the differences between the systems, see the article HT versus KG system: which waste system to choose and why.

Can I save money and buy DN 40 instead of DN 50 for the shower?

Theoretically, DN 40 would be sufficient for a standard shower head with a flow rate of up to 9 l/min, but I do not recommend it in practice. First, most shower traps have a DN 50 outlet, so you would immediately have to reduce to DN 40 after the trap – which is unnecessary. Second, DN 40 clogs much faster with hair and soap, which is a real issue with showers. Third, the price difference between DN 50 and DN 40 is minimal, but the benefits are measurable. Therefore, always choose DN 50 for the shower.

Where exactly should the HT cleaning fitting be placed along the line?

The cleaning fitting should be placed wherever you need access to the pipe in case of a blockage, but where the pipe itself is not visible (inside a wall, behind tiling, in a shaft). Typically: at the beginning of the horizontal branch before the vertical stack (the most common clogging point), on the vertical stack at each floor or direction change, and on the main stack just before it enters the ground. For more information on the use of cleaning fittings and overall system inspection, I recommend the article How to use HT cleaning fittings and inspection fittings for pipe maintenance.

What is the difference between DN and the external diameter in HT piping?

In the HT system, pipes are labeled by their external diameter in millimeters, and the abbreviation DN (Diameter Nominal – nominal diameter) is used in practice as a synonym for the external diameter. So HT DN 110 = HT pipe with an external diameter of 110 mm. The internal (clear) diameter is smaller by the wall thickness (typically 3.2 mm on each side, so DN 110 has a clear diameter of approximately 103 mm). This is important to know when calculating flow capacity and when comparing with standards, where the clear diameter and nominal diameter are sometimes stated differently. A more detailed explanation is provided in the article How to choose the right HT pipes and fittings for the waste system.

Conclusion: the correct diameter will save you years of trouble

Selecting the diameter of HT piping is not a matter of chance or personal preference. It is a precisely defined technical choice based on standards, hydraulics, and practical experience. If you remember only one rule, let it be this: Toilet always DN 110, everything else in the bathroom DN 50, the vertical stack must never be smaller than the largest branch.

The rest is about details, slopes, correct fittings, and accessibility for inspection. There are more articles in this Knowledge Center covering everything from step-by-step installation through connecting fittings to cleaning and system maintenance. If you are unsure about a specific situation in your project, a detailed schematic with real measurements will always help more than a rough estimate.

Do you have a question about this topic?

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