Size the cold and hot water riser with its branch-offs: probable flow by the chosen method, groups used all at once and continuous flows (emergency cooling), a pressure budget from the mains pressure with water meter and height, and size and velocity for every section.

DIN 1988-300 · 6.75 l/s · 600 kPa

Domestic Water Pipe Sizing

06/11/2026

DIN 1988-300 (by building type) · Dwellings, assisted living, senior housing · Copper EN 1057

Simultaneity

Calculation method

DIN 1988-300 (by building type)
V̇S = a·(ΣV̇R)^b − c [l/s] with the constants of the chosen building type.

Building type

Nominal flow per outlet

Outlet	Pipe	Nominal flow (l/s)
Wash basin	CW + HW
Shower	CW + HW
Bath	CW + HW
Kitchen sink	CW + HW
WC cistern	CW
Washing machine	CW
Dishwasher	CW
Hose tap	CW

Flow per pipe. A mixing outlet counts with its full flow in both the cold and the hot water pipe. The values follow the chosen method (Swedish Säker Vatten and DS 439, or DIN 1988-300 table 2) and can be changed to the manufacturer's figures.

Water

Cold water (°C)

Hot water (°C)

Also size the hot water pipes

Pressure and incoming main

Lowest pressure at the connection point (kPa)

Highest static pressure (kPa)

Ask the water utility for the lowest and highest pressure at the connection point. The lowest pressure is used for sizing, the highest for the static pressure check.

Highest allowed static pressure at outlets (kPa)

EN 806-2 gives 500 kPa; follow national rules and the manufacturer's requirements.

Water meter

Meter Δp at Q3 (kPa)

Backflow preventer Δp (kPa)

Filter and other Δp (kPa)

Pressure reducing valve, set pressure (kPa)

Water heater/exchanger Δp (HW) (kPa)

The meter is chosen so that the design flow ≤ Q3, and its loss is taken as Δp at Q3 · (Q/Q3)². 63 kPa is the highest allowed by EN ISO 4064 (class Δp63); the manufacturer's value is often lower. Pressure reducing valve 0 = none.

Pipes and sizing

Pipe system (standard)

Europe's most common metal pipe for drinking water. Exposed runs, risers and plant rooms.

✓ Cold and hot water🌍 EuropeSizes 12×1 – 159×3

Max velocity CW (m/s)

Max velocity HW (m/s)

Usually at most 2 m/s. Hot water in copper: at most about 1.5 m/s to avoid erosion corrosion.

Smallest size allowed

Extra length for fittings (%)

Added to each section's length for bends, tees and valves when the pressure drop is calculated.

Project

File name

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File

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Units

Flow

Pressure

Velocity

Length

Temperature

Power

Flow diagram

Cold water Hot water Force 90° bends Force straight pipes Move with the branch

Drag from the ＋ at a box or a branch-off to build pipes outwards: drop on empty space for a new branch-off, on a load to connect it. Right-click a pipe to add a load or a branch-off. Click a pipe, a branch-off or a load to change it. Drag boxes and branch-offs to move them.

Results per section

Show calculation

Include on printout

Calculation per pipe section

Pipe section	Mass flow [kg/s]	Density [kg/m³]	Inner diameter [mm]	Velocity [m/s]	Re	Friction factor λ	Length (+ fittings) [m]	Δp total [kPa]	Δp per metre [Pa/m]	Size · note
Cold water Incoming cold water → T1	4.698	999.8	60	1.66	76,300	0.0192	10 (+3)	5.73	441	64×2
Cold water T1 → T2	4.028	999.8	60	1.42	65,400	0.0198	15 (+4.5)	6.53	335	64×2
Cold water T2 → T3	3.028	999.8	50	1.54	59,000	0.0203	10 (+3)	6.26	482	54×2
Cold water T3 → T4	1.78	999.8	39	1.49	44,500	0.0216	20 (+6)	16	614	42×1,5
Cold water T4 → Changing room	1.78	999.8	39	1.49	44,500	0.0216	3 (+0.9)	2.39	614	42×1,5
Cold water T3 → T8	1.248	999.8	32	1.55	38,000	0.0224	3 (+0.9)	3.29	842	35×1,5
Cold water T8 → Flats floor 1	0.7417	999.8	25.6	1.44	28,200	0.024	5 (+1.5)	6.32	972	28×1,2
Cold water T8 → T10	1.132	999.8	32	1.41	34,500	0.0229	3 (+0.9)	2.76	708	35×1,5
Cold water T10 → Flats floor 2	0.7417	999.8	25.6	1.44	28,200	0.024	5 (+1.5)	6.32	972	28×1,2
Cold water T10 → T12	0.9784	999.8	25.6	1.9	37,300	0.0225	3 (+0.9)	6.2	1,590	28×1,2
Cold water T12 → Flats floor 3	0.7417	999.8	25.6	1.44	28,200	0.024	5 (+1.5)	6.32	972	28×1,2
Cold water T12 → T14	0.7417	999.8	25.6	1.44	28,200	0.024	3 (+0.9)	3.79	972	28×1,2
Cold water T14 → Flats floor 4	0.7417	999.8	25.6	1.44	28,200	0.024	5 (+1.5)	6.32	972	28×1,2
Cold water T2 → Emergency cooling (mains-water-cooled HX)	0.9998	999.8	25.6	1.94	38,100	0.0224	3 (+0.9)	6.44	1,650	28×1,2
Cold water T1 → Laundry	0.6699	999.8	25.6	1.3	25,500	0.0245	3 (+0.9)	3.17	812	28×1,2
Hot water Water heater → T5	2.85	985.8	50	1.47	144,000	0.0169	5 (+1.5)	2.35	361	54×2
Hot water T5 → T6	2.781	985.8	50	1.44	141,000	0.017	25 (+7.5)	11.2	345	54×2
Hot water T6 → T7	1.755	985.8	39	1.49	114,000	0.0177	20 (+6)	12.9	498	42×1,5
Hot water T7 → Changing room	1.755	985.8	39	1.49	114,000	0.0177	3 (+0.9)	1.94	498	42×1,5
Hot water T6 → T9	1.026	985.8	32	1.29	81,100	0.019	3 (+0.9)	1.92	491	35×1,5
Hot water T9 → Flats floor 1	0.574	985.8	25.6	1.13	56,700	0.0205	5 (+1.5)	3.29	506	28×1,2
Hot water T9 → T11	0.9224	985.8	32	1.16	72,900	0.0194	3 (+0.9)	1.58	405	35×1,5
Hot water T11 → Flats floor 2	0.574	985.8	25.6	1.13	56,700	0.0205	5 (+1.5)	3.29	506	28×1,2
Hot water T11 → T13	0.7853	985.8	32	0.991	62,000	0.0201	3 (+0.9)	1.19	304	35×1,5
Hot water T13 → Flats floor 3	0.574	985.8	25.6	1.13	56,700	0.0205	5 (+1.5)	3.29	506	28×1,2
Hot water T13 → T15	0.574	985.8	25.6	1.13	56,700	0.0205	3 (+0.9)	1.97	506	28×1,2
Hot water T15 → Flats floor 4	0.574	985.8	25.6	1.13	56,700	0.0205	5 (+1.5)	3.29	506	28×1,2
Hot water T5 → Laundry	0.069	985.8	10	0.891	17,400	0.0271	3 (+0.9)	4.13	1,060	12×1

Basis of calculation

Method

DIN 1988-300 (by building type)

Equations

Δp = λ · (L + L_eq)/d · ρv²/2
L_eq = L · 30 %
Δp_total = Δp + Δp_comp
Δp_h = ρ · g · Δh

Friction factor

Colebrook–White (Re ≥ 4000), 64/Re (Re < 2300)

Water properties

IAPWS-95 (CoolProp) 8.0.0

Standards

DIN 1988-300:2012 · EN 806-3 · EN 806-2

Notes

V̇S = a·(ΣV̇R)^b − c [l/s] with the constants of the chosen building type.
Copper EN 1057, ε = 0.0015 mm

Pressure at the loads

Pipe list

FAQ – domestic water pipe sizing

How do you size a domestic water supply pipe?

Add up the design flow rates of all outlets the pipe serves, convert the sum into a probable (simultaneous) flow and pick the smallest size that keeps the velocity below the limit. Then check that the pressure is enough at the worst outlet: the lowest mains pressure minus water meter, backflow preventer, height and pipe friction. The calculator does this for every pipe in the flow diagram.

Calculate it in the tool above

What are loading units and probable flow?

Each outlet has a design flow rate, e.g. 0.2 l/s for a shower and 0.1 l/s for a basin; in EN 806-3 this is expressed as loading units (1 LU = 0.1 l/s). Because not all outlets run at once, the pipe is sized for the probable flow, worked out from the sum with the Swedish probability formula, DS 439 or DIN 1988-300.

Calculate it in the tool above

How do I size the pipe for showers used all at once?

In changing rooms, school showers, laundries and industry everything really runs at the same time. Mark the load “all at once” and the full flow is used. Continuous flows, such as emergency cooling with a mains-water-cooled heat exchanger, are always added in full.

Calculate it in the tool above

What pressure loss can a water supply take?

A flow pressure of about 100 kPa at the outlet is a common requirement, more for some mixers and heat exchangers. Every metre of height costs about 10 kPa, plus the water meter, backflow preventer, filter and pipe friction. The static pressure at an outlet should not exceed 500 kPa (EN 806-2); above that a pressure reducing valve is needed.

Calculate it in the tool above

Can I use EN 806-3 pipe sizing with loading units?

EN 806-3 is the European basis for sizing water installations. The conversion curve of its simplified loading-unit method is not in the calculator yet. Use the Swedish method, DS 439 or DIN 1988-300 (which complements EN 806 in Germany) instead; the flow rates per outlet can be changed to match your data.

Calculate it in the tool above

What velocity should cold and hot water pipes be sized for?

Usually at most 2 m/s in cold water pipes and about 1.5 m/s for hot water in copper to avoid erosion corrosion. Lower velocities may be needed in homes to keep noise down. The limits can be changed in the calculator.

Calculate it in the tool above

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Domestic Water Pipe Sizing