Sizing pipes for drinking water: how to calculate diameter and length

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Pipe sizing for drinking water: how to calculate diameter and length

One of the most underestimated phases of any home plumbing installation is the correct sizing of the piping. Most people focus on the choice of material – copper, plastic or multi-layer – and forget that even the highest quality material will not help if the pipe is oversized or, worse, undersized. The result is then low pressure when showering, hammering in the pipes, excessive noise and in the worst case damage to valves or appliances.

This article will take you through the entire sizing process – from basic concepts through calculation methods to practical examples. We are focusing exclusively on cold and hot drinking water in family homes, apartments and small commercial premises, where the Slovak standard STN EN 806 and related regulations apply.

Why sizing matters more than you think

From practice I know that the biggest problem is not expensive materials or complicated joining – it is the underestimation of hydraulics. I have encountered a case where a new family home used a 12 mm diameter (outer diameter of multi-layer pipe) for the entire hot water distribution. The result? When the tap in the kitchen and the shower were opened at the same time, the pressure in the shower dropped to almost zero. The solution required partial removal of the floor and replacement of all the distribution lines.

On the other hand, I have also seen the opposite extreme – a cabin with two draw-off points and a DN32 pipe, where the water in the network stood still and was not renewed for weeks. The result? Microbiological contamination and a lengthy disinfection of the entire system.

Correct sizing is therefore not only about pressure and flow – it has a direct impact on hygiene, the lifespan of valves and overall energy efficiency. For more information on hygiene and material safety, please read the article Hygiene and material safety of drinking water pipes: what you need to know in this Knowledge Center.

Basic terms and quantities you need to know

Before we get into the actual calculations, let's clarify the terminology. Many laymen confuse terms, which leads to errors in communication with builders or when ordering materials.

DN (Diameter Nominal) – nominal diameter, a standardized labeling system. This is a nominal value, not the actual inner diameter. For example, a DN15 pipe may have an actual inner diameter of 13–16 mm depending on the material and wall thickness.
Outer diameter (d_e) – the actual outer dimension of the pipe including the wall. For a multi-layer pipe 16×2, the outer diameter is 16 mm and the wall thickness is 2 mm.
Inner diameter (d_i) – the actual flow diameter, i.e. d_e minus 2× wall thickness. For a pipe 16×2, it is 16 − 4 = 12 mm.
Flow rate (Q) – the volume of water flowing per unit of time, given in liters per second (l/s) or liters per minute (l/min).
Flow velocity (v) – the speed of water movement in the pipe in meters per second (m/s). For drinking water, the recommended range is 0.5–2.0 m/s.
Pressure loss (Δp) – the drop in pressure due to friction of water against the pipe walls and local resistances (bends, T-pieces, valves), given in kPa or bar.
Working pressure (PN) – maximum allowable operating pressure. For a standard home plumbing system, PN10 (10 bar) applies, which is more than enough, as the pressure in the water supply network rarely exceeds 4–6 bar.

Normative basis: STN EN 806 and calculated flows

The Slovak technical standard STN EN 806 (in full: Technical conditions for water supply equipment within buildings) defines the procedure for designing internal water supply. For sizing, the key part of this standard is part 3, which introduces the concept of calculated flow.

The calculated flow is not the sum of all draw-off points at once – that would be too conservative and would lead to over-sizing. The standard is based on the statistical probability of simultaneous draw-off. The more draw-off points, the smaller the proportion of them that will be open at the same time.

Calculated flows for individual devices (LU – Load Units)

Standard STN EN 806-3 introduces hydraulic load units LU (Load Units), which take into account not only the flow but also the frequency of use of the device. Here are approximate values for common devices:

Device	LU (cold water)	LU (hot water)	Min. flow [l/s]
Basin	1	1	0.10
Shower	2	2	0.15
Bath	3	3	0.20
WC cistern	2	–	0.10
Kitchen sink	1	1	0.10
Dishwasher	1	–	0.10
Washing machine	2	–	0.15
Garden tap (external)	3	–	0.20

From LU to calculated flow: nomogram or formula

The standard provides a nomogram from which the calculated flow Q_calc is read for a given sum of LU. For everyday use in single-family homes, a simplified approximation can be used:

Q_calc [l/s] ≈ K × √(ΣLU)
where K = 0.5 for residential buildings and single-family homes

Example: a single-family house has the following fixtures: 3× sink (3 LU), 2× shower (4 LU), 1× bathtub (3 LU), 2× toilet (4 LU), 1× kitchen sink (1 LU), 1× dishwasher (1 LU), 1× washing machine (2 LU), 1× garden tap (3 LU). Total: ΣLU = 21.

Q_calc = 0.5 × √21 = 0.5 × 4.58 = 2.29 l/s

This is the calculated flow for the main supply to the house. For individual branches, it is calculated separately with only the consumers on that branch.

Choosing the pipe diameter: connecting flow and velocity

Once we know the calculated flow, we can determine the required internal pipe diameter. The relationship between flow, velocity, and cross-section is simple:

Q = v × A, where A = π × (d_i/2)²

From this: d_i = 2 × √(Q / (π × v))

The key parameter is the flow velocity v. For potable water distribution, the following recommended limits apply:

Minimum velocity: 0.5 m/s – below this value, sedimentation, water stagnation, and microbiological problems, especially the risk of Legionella in hot water, may occur.
Maximum velocity in household distribution: 2.0 m/s – above this value, excessive noise, vibrations, and erosive wear of fittings occur.
Maximum velocity in the supply pipe (rising main, main distribution): 1.5 m/s – a more conservative value for branches with longer piping.
Recommended operating velocity: 0.8–1.5 m/s – a compromise between pressure losses and noise.

Practical table: pipe diameter according to flow

The following table lists the maximum recommended flow for common pipe sizes at a velocity of 1.0 m/s (optimum) and 1.5 m/s (maximum for main branches). It also lists the actual internal diameter for multi-layer pipe (wall 2 mm):

Pipe size	di [mm]	Q at 1.0 m/s [l/s]	Q at 1.5 m/s [l/s]	Typical use
16×2 mm	12	0.11	0.17	Connection of 1 consumption point
18×2 mm	14	0.15	0.23	Connection of 1–2 consumption points
20×2 mm	16	0.20	0.30	Distribution branch, 2–4 consumption points
25×2.5 mm	20	0.31	0.47	Main branch of floor, 4–8 consumption points
32×3 mm	26	0.53	0.80	Riser, house supply pipe
HEPWORTH 15 mm	13	0.13	0.20	Connection of 1–2 consumption points
HEPWORTH 22 mm	19.6	0.30	0.45	Main branch, 3–6 consumption points

Pressure losses: how to calculate them and avoid them

Besides flow velocity, pressure loss is the second key parameter. Every meter of pipe and every fitting (elbow, T-piece, reducer, valve) draws off part of the available pressure. If the total pressure loss of the installation exceeds the difference between the pressure in the network and the minimum required pressure at the device, the system simply does not work properly.

Friction in pipes: Darcy-Weisbach equation

Linear (friction-induced) pressure loss is calculated according to the formula:

Δp_L = λ × (L / d_i) × (ρ × v² / 2)

where:
λ = friction coefficient (for smooth plastic pipes ≈ 0.02–0.03)
L = length of section [m]
d_i = inner diameter [m]
ρ = density of water ≈ 1000 kg/m³
v = flow velocity [m/s]

For a quick estimate, you can use an approximate value of pressure loss in Pa per meter of pipe. At a velocity of 1.0 m/s, the following applies for multi-layer pipes:

16×2 (di=12 mm): ≈ 250–350 Pa/m (2.5–3.5 mbar/m)
20×2 (di=16 mm): ≈ 120–180 Pa/m
25×2,5 (di=20 mm): ≈ 70–100 Pa/m
32×3 (di=26 mm): ≈ 35–55 Pa/m

Local resistances: equivalent lengths

Fittings and valves are converted into an equivalent length of straight pipe for calculation purposes. Approximate values:

90° elbow: 0.5–1.5 m equivalent length (depends on diameter)
T-piece (flow through branch): 1.0–3.0 m
T-piece (flow straight through): 0.2–0.5 m
Ball valve (fully open): 0.1–0.3 m
Angle valve: 3.0–8.0 m
Check valve: 2.0–5.0 m

Practical rule: in a typical family house, add 20–30% to the measured pipe length as a lump sum for local resistances. For more complex installations with many elbows and valves, even 40–50%.

Comprehensive sizing procedure step by step

Practical example: sizing the layout for a family house

Let's look at a specific case that we commonly solve. A two-story family house, ground floor + first floor, supplied from the public water supply. Pressure in the network at the house connection: 3.5 bar (350 kPa). Floor height: 2.8 m, roof height: 0 (flat). The connection enters the technical room on the ground floor.

Draw-off points on the ground floor: kitchen (sink + dishwasher), WC (flush toilet + washbasin), washing machine in the utility room.

Draw-off points on the first floor: bathroom 1 (bath + washbasin + shower), bathroom 2 (shower + washbasin), WC (flush toilet + washbasin).

Outdoor draw-off: garden tap on the north side of the house.

Step 1–2: Calculation of LU and Q_výp

Ground floor: sink (1 LU cold + 1 LU hot), dishwasher (1 LU cold), WC flush toilet (2 LU cold), washbasin (1 LU cold + 1 LU hot), washing machine (2 LU cold). Total ground floor: cold = 8 LU, hot = 2 LU.

First floor: bath (3+3), shower 1 (2+2), washbasin 1 (1+1), shower 2 (2+2), washbasin 2 (1+1), WC flush toilet (2 cold), WC washbasin (1+1). Total first floor: cold = 12 LU, hot = 10 LU.

Outdoor tap: 3 LU cold.

Total cold water: ΣLU = 8 + 12 + 3 = 23 LU

Q_výp,SV = 0.5 × √23 = 0.5 × 4.80 = 2.40 l/s – for the main supply to the house.

Total hot water: ΣLU = 2 + 10 = 12 LU

Q_výp,TV = 0.5 × √12 = 0.5 × 3.46 = 1.73 l/s – for the hot water supply pipe.

Step 3: Selection of diameter of the main supply pipe

For Q = 2.40 l/s and maximum velocity 1.5 m/s:
d_i = 2 × √(2.40×10⁻³ / (π × 1.5)) = 2 × √(0.000509) = 2 × 0.02257 = 0.04514 m = 45.1 mm

This corresponds to at least DN40 (inner diameter ≈ 41 mm). For the supply, we would therefore use a pipe of 50×4.5 mm or steel/iron DN50 according to local regulations. The house connection is usually DN32–DN50 according to the contract with the water company.

Steps 4–5: Distribution per floor and to the bathrooms

For the hot water branch to the first floor (ΣLU hot = 10 LU, Q = 0.5×√10 = 1.58 l/s) at a velocity of 1.2 m/s:
d_i = 2 × √(1.58×10⁻³ / (π × 1.2)) = 2 × 0.02049 = 40.9 mm → DN32–DN40 is sufficient.

For connecting one shower or sink (Q = 0.15 l/s) at a speed of 1.0 m/s:
d_i = 2 × √(0.15×10⁻³ / (π × 1.0)) = 2 × 0.00691 = 13.8 mm → sufficient multilayer pipe 16×2 mm (di=12 mm at speed ≈1.3 m/s) or 18×2 mm (di=14 mm, speed ≈ 0.97 m/s – ideal).

For floor distribution pipes serving 3–4 devices (Q ≈ 0.35–0.50 l/s), it is suitable to use multilayer pipe 20×2 mm (di=16 mm, Q_max at 1.5 m/s = 0.30 l/s) or preferably 25×2.5 mm for greater comfort.

Special situations: elevation difference and pressure conditions

Along with friction in the pipes, elevation must also be considered. Each meter of elevation reduces pressure by 9.81 kPa ≈ 0.1 bar. In a two-story house with a 5 m elevation difference between the connection and the highest point of use (a sink on the upper floor), you lose 0.5 bar just to overcome the elevation.

Example: Network pressure = 3.5 bar. Elevation difference = 5 m → pressure difference = 0.5 bar. Pressure loss in the entire pipe run (20 m, with local resistances equivalent to +6 m = total 26 m) at a diameter of 20×2 and speed of 1.2 m/s ≈ 140 Pa/m × 26 m = 3 640 Pa = 0.036 bar. Minimum required pressure before the shower = 0.5 bar. Total balance: 3.5 – 0.5 – 0.036 = 2.96 bar – more than sufficient.

Problems arise with low-pressure networks (under 2 bar) or large elevations (multi-story buildings over 5 floors). In such cases, hydraulic pressure boosting (pump) or pressure tank is necessary. This article focuses on standard residential installations, where such problems usually do not occur.

Copper pipe HEPWORTH: sizing and specifics

The principles of sizing are the same for all materials, but copper pipes have a few differences that should be considered.

Copper pipe has a smoother inner surface (roughness ≈ 0.0015 mm compared to ≈ 0.007 mm for polyethylene), which means a slightly lower friction coefficient λ at the same speed. The practical impact on pressure loss is minimal at normal speeds (up to 1.5 m/s).

More important is the fact that copper pipe HEPWORTH 15 mm has an outer diameter of 15 mm and an inner diameter of ≈ 13 mm (wall thickness 1 mm), which is more than multilayer 16×2 (di=12 mm). Therefore, for the same capacity, copper pipe requires a slightly smaller nominal size. HEPWORTH 22 mm has an inner diameter of ≈ 19.6 mm – very similar to multilayer 25×2.5.

A detailed comparison of the properties of both materials can be found in the article Multilayer pipe vs. HEPWORTH: which is better for your system.

Pipe lengths: how to measure and order materials correctly

Sizing also includes determining the required pipe length, which is just as important as selecting the diameter. Over-sizing the length increases costs, while under-sizing causes delays and rework.

Measuring actual routes

When planning, always measure pipe routes along real paths, not straight lines. Pipes run through walls, floors, and ceilings – consider the following:

Horizontal routes in the floor or ceiling – measure along the plan
Vertical risers – floor height plus transitions through the structure
Bends and details – for multilayer pipe in the ceiling, calculate with curves, not elbows (bend radius 5–8× outer diameter)
Allowance for errors and adjustments: at least 10% of the total length

Material list example for a bathroom on the upper floor

Bathroom on the upper floor: bathtub, shower, sink. The riser SV/TV comes into the bathroom from below (through a penetration in the floor). The distribution runs in the floor structure.

Riser SV/TV: 2× multilayer 20×2, length 2.9 m (floor height 2.8 m + transitions)
Distribution loop in the bathroom SV/TV: 2× multilayer 16×2, total length ≈ 9 m (including 10% reserve)
Fittings: 4× T-piece 16×16×16, 2× T-piece 20×16×20, 6× elbow 16 90°, 2× reducer 20→16
Fittings: 3× angle valve (under the bathtub, shower, sink), 1× ball valve SV for the entire bathroom

Most common sizing errors and how to avoid them

From practice, I have observed the following recurring errors:

Using the same diameter for the entire installation – “it is simpler” – the result is either an under-sized main branch or over-sized connections to the devices.
Ignoring calculated flows – adding all maximum flows without a reduction factor leads to unnecessarily large diameters and slow flow, which is a hygiene issue.
Forgetting pressure losses in hot water – hot water circulation has its own hydraulic parameters that need to be sized separately.
Underestimating local resistances – in a bathroom with many elbows and T-pieces, local resistances can account for 50–80% of the total pressure loss in the branch.
Incorrect length measurements – measuring along a straight line instead of the actual route.
Using the same diameter for a garden tap as for device connections – a garden tap has a high flow rate (0.20 l/s) and must not share a narrow branch with other connections.

Dimensioning the hot water circulation loop

Many people forget that a family house with hot water distribution lines longer than 3–4 m needs a circulation loop to reduce the waiting time for hot water. The circulation pipe (so-called return line) is usually one size smaller than the supply line – if the supply is 20×2, the circulation goes in 16×2.

The circulation flow rate is dimensioned according to the pipe heat losses, not according to the consumption flow rates. For a typical family house with insulated piping, it ranges between 0.05–0.15 l/s, which for a 16×2 (di=12 mm) diameter means a speed of only 0.4–1.3 m/s – within the standard. A circulation pump is typically small (e.g. Wilo Star-Z, power 5–30 W).

The topic of pressure classes and temperatures in hot water distribution is discussed in detail in the article Pressure classes PN10 and temperatures +70°C vs. +95°C: what they mean for your distribution.

Frequently asked questions (FAQ)

What pipe diameter is sufficient for a single consumption point (basin, shower)?

For a single consumption point with a flow rate of 0.10–0.15 l/s, a multilayer pipe 16×2 mm or copper HEPWORTH 15 mm is sufficient. The flow velocity will be in the range of 0.9–1.3 m/s, which is optimal. If the connection is longer than 5–6 m or there are multiple devices on the branch, we recommend 18×2 or 20×2.

Can I use the same diameter for cold and hot water?

Yes, the calculated flow rates for both circuits are dimensioned in the same way. Hot water usually has slightly lower calculated flow rates (WC, garden taps and most washing machines are not connected to it), so the supply pipe for hot water can be one size smaller than for cold water. Connections to taps are always the same diameter for cold and hot water.

How can I find out what pressure I have in the water supply network?

The most reliable way is to measure it with a pressure gauge connected to a household tap (e.g. a garden tap). An approximate value can be provided by the local water utility – for most Slovak cities, the static pressure is 2.5–5.0 bar. However, the more important is the dynamic pressure during consumption, which can be 0.5–1.0 bar lower.

Is multilayer piping less "capacitive" than copper due to the aluminium layer?

No. The aluminium layer is inside the pipe wall, the inner surface is polyethylene (PE-X or PE-RT). The hydraulic properties of multilayer piping are comparable to copper – the friction coefficient is slightly higher, but at normal flow rates (up to 1.5 m/s) the difference in pressure loss is less than 10–15%, which is negligible for dimensioning.

Do I need to do a hydraulic calculation for a bathroom renovation in an apartment?

For a simple replacement of old pipes with new ones (of the same diameter), a formal calculation is not mandatory, but we recommend at least

Do you have a question about this topic?

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