A pipe that is too small produces excessive flow velocities with noise, high pressure drop, and long-term erosion damage; one that is too large not only costs more but also promotes bacterial growth in hot water systems through stagnation. The pipe sizing calculator uses the basic hydraulic formula to find the optimal internal diameter and assigns it to the appropriate nominal size.
Step by Step: How to Use the Pipe Sizing Calculator
- Select the application: Heating, cold drinking water, hot drinking water, or wastewater — each application has its own recommended flow velocities.
- Enter the flow rate: For heating, from the heat load calculation: Q [L/h] = heat load [kW] × 860 ÷ temperature spread [K]. For drinking water, add up the peak flow for all draw-off points per DVGW W 553.
- Choose the target flow velocity: Heating 0.3–0.8 m/s; cold drinking water 0.5–2.0 m/s; hot drinking water 0.5–1.0 m/s (DIN 1988). Wastewater min. 0.5 m/s for self-cleaning.
- Let the calculator determine the internal diameter: Formula: D = √(4 × Q / (π × v)). The calculator outputs the minimum internal diameter in mm.
- Select and verify the nominal size: Round up to the next larger standard size (DN15, DN20, DN25, DN32, DN40 …) and check the resulting flow velocity again.
Practical Examples
Example 1 – Heating flow pipe: Heat load 12 kW, spread 10 K → Q = 12 × 860 ÷ 10 = 1,032 L/h = 0.000287 m³/s. At v = 0.5 m/s: D = √(4 × 0.000287 / (π × 0.5)) = 0.027 m = 27 mm → select DN25 (outer diameter 28 mm).
Example 2 – Drinking water in an apartment block: 6 units, peak flow 0.3 L/s per DVGW. At v = 1.5 m/s: D = √(4 × 0.0003 / (π × 1.5)) = 16 mm → DN20 (copper 22×1 mm) is sufficient.
Example 3 – Industrial cooling water: 50 m³/h = 0.0139 m³/s, max. v = 1.0 m/s (erosion protection). D = √(4 × 0.0139 / (π × 1.0)) = 0.133 m → DN150.
Pipe Diameter Calculation
Formula: D = √(4 × Q / (π × v)). Recommended flow velocities: heating 0.3–0.8 m/s; drinking water 0.5–2 m/s; wastewater 0.5–1.5 m/s. Heating circuit 1,000 L/h at max. 0.5 m/s → D = 26.6 mm → select DN25.
Frequently Asked Questions (FAQ)
Why is a lower maximum velocity recommended for hot drinking water?
Hot water accelerates corrosion on copper surfaces. Above 50 °C, an upper limit of 1.0 m/s applies to prevent pitting caused by turbulence. Stainless steel and multilayer composite pipes allow slightly higher values.
How do I calculate the flow rate for a heating system without a formal heat load calculation?
As a rough estimate, well-insulated homes require approximately 50–75 W/m² of floor area. A 150 m² house needs around 7.5–11 kW heat load. Precise values come from a heat load calculation per DIN EN 12831.
What is the difference between nominal size DN and outer diameter?
DN is a standardised reference number with no direct dimensional meaning. A copper pipe DN15 has an outer diameter of 18 mm; a steel pipe DN15 has 21.3 mm — the internal diameter varies depending on wall thickness and material.
