Velocity and Hazen-Williams friction loss by flow for DN25 pipe. Loss values are in bar per 100 m, calculated on an internal diameter of roughly 27.3 mm.
Typical use: Branch end; feeding one or two sprinklers.
Velocity and friction loss table (DN25)
| Flow (L/min) | Velocity (m/s) | C=100 | C=120 | C=140 |
|---|---|---|---|---|
| 50 | 1.42 | 1.701 | 1.214 | 0.913 |
| 100 | 2.85 | 6.133 | 4.377 | 3.291 |
| 150 | 4.27 | 12.984 | 9.267 | 6.967 |
| 200 | 5.69 | 22.108 | 15.778 | 11.863 |
| 300 | 8.54 | 46.808 | 33.407 | 25.118 |
| 400 | 11.39 | 79.699 | 56.881 | 42.768 |
The C factor represents internal pipe roughness. Standards give different C values by pipe material and system type; a lower C for dry systems is common.
The calculation
Hazen-Williams in metric form: p = 6.05 × 105 × Q1.85 / (C1.85 × d4.87), where p is bar/m, Q is L/min and d is the internal diameter in mm.
The relation has two consequences:
- Doubling the flow raises the loss by roughly 3.6 times (21.85).
- Going up one pipe size cuts the loss dramatically; loss falls with the 4.87 power of diameter.
Internal diameter matters, not nominal size. The table values assume an internal diameter of roughly 27.3 mm. Pipes of a different wall thickness have a different bore and a correspondingly different loss; a precise calculation must use the pipe's own technical data.
Velocity check
High velocity brings noise, water hammer and long-term erosion. The velocity column lets you see at a glance whether the chosen size stays in a reasonable range. Where the velocity comes out too high, the answer is a larger diameter.
How to use it
- Determine the flow through this pipe section in the hydraulic calculation.
- Read the bar per 100 m loss from the table at the appropriate C value.
- Scale it to the actual length of the section.
- Add the equivalent lengths of fittings and valves.
- Add the elevation difference as static pressure.
Frequently Asked Questions
What C factor applies to DN25 pipe?
The value the standard gives for the pipe material and system type. The table has separate columns for C=100, 120 and 140; a lower C for dry systems is common.
What happens to the loss if flow doubles?
It rises by roughly 3.6 times (2^1.85). Flow increases therefore affect loss more than expected.
Is the nominal size the same as the bore?
No. The table assumes an internal diameter of roughly 27.3 mm; a different wall thickness changes the bore and the loss.
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Download MEP Calc on the App StoreNFPA 13 (2025) · BS EN 12845:2015+A1:2019. Values are calculated from Hazen-Williams using an approximate internal diameter; a precise calculation must use the pipe's technical data and the C factor the standard requires.