What PPR pipe diameter do I need – calculation based on pressure and flow
Why the choice of PPR pipe diameter matters more than you think
One of the most common mistakes I see in both DIY and professional installations is underestimating the importance of selecting the right PPR pipe diameter. A customer comes in saying they "need a pipe for water," shows their thumb, and assumes that's enough. It's not. The wrong pipe diameter causes problems that usually become apparent only later—for example, when you turn on the shower and the kitchen tap at the same time, the pressure in the shower drops almost to zero. Or, on the contrary—you unnecessarily overload the family budget with oversized piping, thinking "the bigger, the better." That's not true either.
In this article, we will look at the whole issue systematically—from basic hydraulics through practical nomograms and tables to real-life examples from apartment and house installations. You don't need to be an engineer to understand the principle. A little patience and common sense are enough.
Basic terms you can't do without
Before we get into the calculations, let's clarify the terminology. PPR pipes are marked by their nominal outer diameter (DN or OD – outer diameter) and wall thickness (SDR or directly in millimeters). Plumbers usually refer to a "20," "25," or "32"—these are the outer diameters in millimeters. The inner diameter (bore, ID – inner diameter) is the one that determines how much water can flow through the pipe and depends on the wall thickness.
For example, a PN20 PPR pipe with an outer diameter of 25 mm has a wall thickness of 4.2 mm, so the bore is only 16.6 mm. A PN10 pipe with the same outer diameter of 25 mm has a wall thickness of only 2.3 mm, so the bore is 20.4 mm. That's a huge difference in flow capacity! Therefore, in calculations, we always work with the bore, not the outer diameter.
Pressure and flow—two key parameters
The whole choice of diameter is based on two quantities:
- Flow rate (Q) – the amount of water that must flow through the pipe per unit of time. It is given in liters per second (l/s), liters per minute (l/min), or cubic meters per hour (m³/h).
- Pressure (p) – the operating pressure in the network, but mainly the pressure loss that we can "allow" on a given section without degrading the comfort at the outlet.
Also important is the flow velocity (v), which is a derived quantity. The ideal flow velocity in a home plumbing system is 0.5 – 1.5 m/s, up to a maximum of 2.0 m/s. If the pipe is too narrow, the water flows faster—this causes excessive noise (hissing in the walls) and high pressure losses. If the pipe is unnecessarily large, the water moves slowly, stagnates, and in hot water, this promotes bacterial growth (legionella).
Flow rate calculation—how much water your installation needs
Before you find out what pipe diameter you need, you must know how much water will flow through the pipe. This is often the first panic attack for DIYers—"I can't measure that." But it can be calculated. There is a simple method based on the design flow rates of individual devices.
Design flow rates (DU) of common household devices
Each device has a so-called nominal design flow rate (DU – Design Unit or directly in l/s). Here are the most common values according to standards (approximately, specific values may depend on the specific faucet system):
| Device | Cold water DU [l/s] | Hot water DU [l/s] |
|---|---|---|
| Basin faucet | 0.10 | 0.10 |
| Shower faucet | 0.15 | 0.15 |
| Bath faucet | 0.20 | 0.20 |
| Kitchen sink | 0.20 | 0.20 |
| Toilet cistern | 0.10 | – |
| Washing machine | 0.20 | 0.20 |
| Dishwasher | 0.15 | 0.15 |
| Garden hose / outlet | 0.30 | – |
These values are not simply added up, because not all devices are operating simultaneously. A so-called simultaneity factor is used or the method according to the standard STN EN 806. For DIY enthusiasts, a simplified approach is sufficient: assume that a maximum of 2–3 devices in the apartment will be operating at the same time (e.g. a shower + WC flush + sink). The sum of their DU values will give you the calculated flow rate for that section.
Basic hydraulic formula – pipe diameter from flow and velocity
Once you know the calculated flow rate, the next step is trivial. From the relation Q = v × S (where S is the cross-section of the pipe), we express the required internal diameter (d) of the pipe:
d = 2 × √(Q / (π × v))
Where: Q is the flow rate in m³/s (watch out for the units!), v is the velocity in m/s, π ≈ 3.14159. The result d is in meters, which we convert to millimeters.
Practical example: You have a shower (DU = 0.15 l/s = 0.00015 m³/s) and you want the water to flow at a speed of 1.0 m/s.
d = 2 × √(0.00015 / (3.14159 × 1.0)) = 2 × √(0.0000477) = 2 × 0.00691 = 0.01382 m ≈ 13.8 mm
Clearance of 13.8 mm – the nearest PPR pipe with sufficient clearance is DN20 (clearance ~16 mm at PN20). This is a standard solution for individual outlet branches in an apartment.
Table of recommended PPR pipe diameters according to flow
For those who do not want to calculate, here is a practical table of recommended diameters for common installations. The values apply to PN20 pipes (the most common pressure class for households) at a flow velocity of 0.8–1.2 m/s:
| Flow Q [l/s] | Flow Q [l/min] | Recommended outer diameter | Clearance PN20 [mm] | Typical use |
|---|---|---|---|---|
| up to 0.10 | up to 6 | DN15 (ext. ∅20mm) | 13.2 | WC cistern, single outlet |
| 0.10 – 0.20 | 6 – 12 | DN20 (ext. ∅25mm) | 16.6 | Shower, sink, sink |
| 0.20 – 0.40 | 12 – 24 | DN25 (ext. ∅32mm) | 21.2 | Apartment distribution, 2–3 devices |
| 0.40 – 0.70 | 24 – 42 | DN32 (ext. ∅40mm) | 27.2 | Apartment riser, house – main distribution |
| 0.70 – 1.20 | 42 – 72 | DN40 (ext. ∅50mm) | 34.4 | Larger house, riser of a small apartment block |
| 1.20 and more | 72+ | DN50 (ext. ∅63mm) | 44.0 | Main riser pipe, multifunctional |
Remember: this table is approximate. In practice, pressure losses at fittings, valves, pipe lengths, and local hydraulic conditions (network pressure) must also be considered.
Pressure loss – a critical factor most people ignore
Flow rate and velocity are only part of the story. Equally important is the pressure loss (ΔP) that occurs when water flows through pipes. The longer the pipe and the smaller the diameter, the greater the pressure loss. If the pressure loss is too high, you may have a correctly sized pipe at the end of the run, but the pressure at the outlet will be insufficient.
As a rough guide: in residential buildings, the total pressure loss in the supply pipe from the riser to the most distant outlet should not exceed 0.1–0.2 MPa (1–2 bar). A typical domestic water pressure is 0.3–0.5 MPa (3–5 bar), and the pressure at the outlet should not drop below 0.1 MPa.
Simplified pressure loss calculation (Darcy-Weisbach)
For a straight pipe section, the following applies:
ΔP = λ × (L / d) × (ρ × v²) / 2
Where: λ is the friction coefficient (for PPR pipes under turbulent flow typically 0.02–0.035), L is the length of the section in meters, d is the inner diameter in meters, ρ is the density of water (≈ 1000 kg/m³), v is the velocity in m/s. The result ΔP is in Pa, divided by 100 000 to get MPa.
Practical example – residential installation: You have a shower in a bathroom located 8 meters from the riser, PPR pipe DN20 PN20 (inner diameter 16.6 mm = 0.0166 m), flow rate 0.15 l/s, velocity 0.7 m/s.
ΔP = 0.025 × (8 / 0.0166) × (1000 × 0.7²) / 2 = 0.025 × 481.9 × 245 = 2 950 Pa ≈ 0.030 bar
This is a completely acceptable value. Problems arise when you have long runs (20–30 m) or an excessively small diameter. In such cases, you must increase the pipe diameter. In addition to friction losses in the pipes, you also need to account for so-called local resistances – elbows, tees, reducers, and valves. In practice, these losses are estimated to be 30–50% of the pipe losses (so-called equivalent length).
Practical scenarios – real installations step by step
Scenario 1: Full apartment renovation (2-room, 65 m²)
This is the most common case we encounter. The apartment has a bathroom (shower + sink + WC) and a kitchen (sink + washing machine). The riser is in the entrance hall, and we run the supply lines from there.
Design flow rate for an apartment layout: we consider simultaneous use of the shower (0.15 l/s) + kitchen sink (0.20 l/s) + WC (0.10 l/s) = maximum 0.45 l/s. Considering a simultaneity factor of approximately 0.7, we get a design flow rate of ~0.30 l/s. For this flow rate and a run length of 6–8 m, DN25 (ext. ∅32 mm) is sufficient for the main apartment branch.
For individual branches to fixtures: each branch to the shower/sink/sink DN20 (ext. ∅25 mm). For the WC cistern DN15 (ext. ∅20 mm). I recommend installing a ball valve on each branch to allow for isolation without turning off the entire apartment. On the atria.sk website, you can find for example a ball valve with a 50 mm diameter suitable for main shut-off valves in apartment supply lines with larger diameters.
Scenario 2: Family house with two bathrooms and a garden
This is a more demanding case. The house has a ground floor (bathroom + kitchen) and a first floor (bathroom + WC), plus a garden tap. The total design flow rate including the garden can reach 0.6–0.8 l/s with simultaneous use. The main supply branch from the house water meter DN32 (ext. ∅40 mm). The riser to the first floor DN25 (ext. ∅32 mm) or also DN32, depending on the length. Branch lines to individual fixtures DN20 (ext. ∅25 mm), taps DN15 (ext. ∅20 mm).
For the garden tap, where a hose is used, it is also important to consider pressure surge when closing quickly – so it is good to have a proper ball valve on this branch, not a cheap pressed fitting.
Scenario 3: Bathroom renovation – only pipe replacement
The customer is replacing only the pipes in the bathroom, the riser remains. We are usually dealing with minimal lengths (2–4 m), and pressure losses are negligible. The main criterion is compatibility with existing connections. In most cases, DN20 for all branches, with DN15 reductions on the taps. Be careful when transitioning from DN15 to ½ inch thread (G ½") for taps – there are standard PPR fittings with metal threaded inserts available for this purpose.
The impact of pipe pressure class (PN) on inner diameter
One thing many people overlook: when selecting the diameter, you must always also consider the pressure class (PN). The same outer diameter can have significantly different inner diameters:
| Outer diameter | Pressure class | Wall thickness [mm] | Inner diameter [mm] | Cross-section [cm²] |
|---|---|---|---|---|
| ∅25 mm | PN10 | 2.3 | 20.4 | 3.27 |
| ∅25 mm | PN16 | 3.5 | 18.0 | 2.54 |
| ∅25 mm | PN20 | 4.2 | 16.6 | 2.16 |
| ∅32 mm | PN10 | 2.9 | 26.2 | 5.39 |
| ∅32 mm | PN16 | 4.4 | 23.2 | 4.23 |
| ∅32 mm | PN20 | 5.4 | 21.2 | 3.53 |
For cold water in the household, PN10 or PN16 is sufficient. For hot water (domestic hot water circuit), at least PN16 is required, and for higher temperatures (above 60 °C) or higher pressures, we recommend PN20. For more information on this topic, see the article Temperature and pressure resistance of PPR pipes – what you need to know before buying in the Knowledge Centre.
When to increase the diameter and when it is unnecessary
From practice, I know that customers sometimes want "better bigger, just to be sure". This makes sense only under certain conditions. Here is an overview of situations when increasing the diameter is really necessary, and when it is a waste of money:
Increasing the diameter makes sense when:
- The length of the pipeline is significantly greater than 10 m (pressure losses increase linearly with length)
- There are many fittings, valves, and other components on the route
- The network has low pressure (less than 2.5 bar) – in this case, every bar is valuable
- You plan to connect additional devices in the future
- It is a hot water circulation loop, where slow flow is desired (prevention of legionella)
A larger diameter does not help, when:
- The pressure in the network is sufficient (over 3 bar) and the pipelines are short
- The device has its own pressure regulator (e.g., a thermostatic shower unit)
- It is a single outlet – increasing from DN15 to DN20 will not bring a visible effect
- You want to save on hot water – a larger pipe holds more cold water, and it takes longer for hot water to arrive
Measuring and cutting – practical preparation of pipes
Once you have designed the system, you need to prepare the pipes to size. Cutting PPR pipes may seem trivial at first glance, but an improperly cut pipe can cause problems with the joint tightness. Always cut straight – a slanted cut means the pipe will not sit evenly in the fitting and the joint will be weakened. The easiest way is with special pipe cutters. On the atria.sk website, you can find standard pipe cutters up to 63 mm, which handle the standard PPR pipe sizes used in households and smaller commercial installations. How to use these cutters and how to proceed with cutting and welding is described in detail in the article How to properly use PPR pipe cutters and a polyfusion welder in the Knowledge Centre.
For larger diameters (from DN50 and above), I recommend using a pipe saw or orbital cutters – standard cutters may compress the larger pipe and the cut will not be straight. Always smooth the inner and outer edges after cutting – a file or scraper will prevent sharp edges from interfering with the seal during welding.
Most common mistakes in selecting the diameter and their consequences
Over the years of practice, I have seen several recurring mistakes. It is worth mentioning them, as some are only discovered during commissioning or after years of use:
1. The entire apartment on DN15 (∅20 mm). The customer bought the cheapest pipes and found out that when multiple taps are open at the same time, the pressure in the shower drops to nothing. The solution is a complete replacement, which is more expensive than if they had bought the right pipes from the start.
2. Forgetting about pressure losses in fittings. The project passed the calculation, but the customer had fifteen elbows, three T-pieces, and two reducers on a short route. Each fitting adds equivalent friction. The pressure at the end of the route was insufficient.
3. Confusing the outer and inner diameter. The customer designed the system "according to the inner diameter," but ordered the outer diameter. The difference between PN10 and PN20 at ∅25 mm is 4.2 mm in the wall, which is almost 4 mm in the internal diameter. When ordering PPR pipes, always specify the outer diameter and pressure class.
4. Ignoring hot water circulation. In a family house with long domestic hot water distribution lines, a circulation pipe is a necessity. This pipe is dimensioned differently – not according to flow, but according to heat losses. Usually DN15 or DN20 with a separate circulation pump is used.
5. Underestimating the garden outlet. A garden hose at full pressure has a flow of 0.3–0.4 l/s. If the garden outlet is at the end of a long route with a small diameter, the whole house suffers during watering. Always solve the garden outlet either with a separate branch of sufficient diameter or with a pressure regulator.
Practical nomogram – quick determination of diameter without a calculator
For those who need a quick orientation in the field without calculations, here is a verbal summary of the nomogram logic used by experienced plumbers:
- One outlet (basin, WC, shower, sink): DN15 or DN20, depending on the distance
- A branch for 2–4 devices in one room: DN20 or DN25
- Main apartment distribution (apartment up to 100 m²): DN25 or DN32
- Riser for 2–4 apartments: DN32 or DN40
- Main supply line (family house): DN32, larger houses DN40
- Riser of a small apartment building (5–10 apartments): DN50 or DN63
This rule of thumb works for standard pressures (3–5 bar) and standard lengths (up to 15 m). In extreme cases, always do the calculation. The article How to choose the right PPR piping system for
Not sure how to decide or are you dealing with a specific situation in your home? Write to us – we are happy to help.Do you have a question about this topic?