Two limits, not one
Most people size pipes by velocity alone, and it is a reasonable first check: too fast and a pipe gets noisy and erodes, too slow and you are paying for metal you do not need. But velocity hides something. Friction losses depend on the diameter as well, and in smaller pipes they climb steeply at velocities that still look comfortable.
Take 22 m³/h of water at 70 °C in steel pipe. DN80 carries it at about 1.3 m/s, well inside a 1.5 m/s limit. Yet it loses close to 200 Pa per metre, which over a 300 m run means about 0.6 bar for friction alone. DN100 drops the velocity to 0.74 m/s and the friction to about 50 Pa/m. If you design heating mains to 100 or 150 Pa/m, the velocity check passes DN80, and the pressure drop check is what tells you to go up a size.
How the calculator works
For each standard size in the chosen series it computes the velocity from the flow and the inside diameter, the Reynolds number from temperature-corrected water viscosity, the friction factor (Swamee-Jain approximation of Colebrook, 64/Re for laminar flow), and the pressure drop per metre with the Darcy-Weisbach equation:
The recommended size is the smallest one that meets both limits. The table shows that size and its neighbours, with the failing values in red, so you can see how much margin you have and what one size up or down would mean.
Typical starting values
| Application | Velocity, m/s | Pressure drop, Pa/m |
|---|---|---|
| District heating and plant heating mains | 1.0 – 2.0 | 50 – 150 |
| Heating and chilled water inside buildings | 0.5 – 1.5 | 100 – 300 |
| Small branches near occupied rooms | up to 0.7 | 100 – 200 |
| Cold water supply | 1.0 – 2.0 | 200 – 500 |
These ranges are common in practice but differ between countries and companies. Long mains that run all year usually justify lower limits, because every pascal saved is paid back in pump energy for decades.
Don't know the flow yet? Get it from the heat load and temperatures with the heating flow rate calculator. Once you have picked a size, check the whole run including fittings with the pipe pressure drop calculator, and see what the pumping will cost each year with the pump energy calculator.
Frequently asked questions
What velocity limit should I use?
For heating and chilled water mains, 1 to 1.5 m/s is a common starting point; small pipes inside buildings are often kept below 0.7 to 1 m/s to avoid noise. Cold water supply lines are frequently sized for up to 2 m/s. These are typical starting values, not rules: check local codes and your company standards.
Why limit pressure drop per metre as well as velocity?
Velocity alone does not tell you how much pumping energy a pipe will cost. Small pipes reach high friction losses at velocities that still look acceptable. A limit in pascals per metre keeps the total pressure drop of a long run, and therefore the pump size and energy bill, under control.
Which pipe sizes are used?
Steel uses Schedule 40 inside diameters (ASME B36.10), PE uses PE100 SDR11 pipes, and copper uses common EN 1057 sizes. Inside diameters of real pipes vary between standards and manufacturers, so check the exact bore of the pipe you will buy.
Is the smallest pipe that passes always the best choice?
Not always. The smallest pipe has the lowest purchase cost but the highest pumping cost over its life. For pipes that run many hours a year, going one size up often pays back. The table shows the neighbouring sizes so you can compare.
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