Pipe Pressure Loss Calculator

Pipe head loss: h_f = f·(L/D)·(v²/(2g))

Friction head loss, measured in meters of water column, comes from the Darcy–Weisbach equation: h_f = f·(L/D)·(v²/(2g)). Here f is the Darcy friction factor (read off the Moody diagram), L the length, D the internal diameter, v the mean velocity and g = 9.81 m/s². What sets f is the Reynolds number Re together with the relative roughness ε/D. A few reference values: smooth PVC ε ≈ 0.0015 mm, commercial steel ε ≈ 0.045 mm, ductile iron ε ≈ 0.25 mm. For water specifically, you can fall back on the empirical Hazen–Williams formula h_f = 10.67·L·(Q/C)^1.852/D^4.87. Brazilian standard NBR 5626 caps the residual pressure drop at less than 8 mwc between the tank and the most unfavorable point.

Applications

Sizing booster pumps for buildings, laying out rural irrigation networks, running residential plumbing for cold and hot water under NBR 5626, designing pool filter circuits and industrial process lines.

FAQ

How does NBR 5626 affect my house? Every pipe has to reach the tap with at least 1 mwc of residual pressure, and the total loss from the upper tank to the tap has to stay under 8 mwc. Cross that line and your shower turns into a dribble.

Hazen–Williams or Darcy–Weisbach? Hazen–Williams is the simpler one, valid for water near 15 °C in turbulent flow, which is why water utilities lean on it. Darcy–Weisbach handles any fluid in any regime and gives you more accuracy.

Why does PVC outperform steel? Its lower roughness (ε ≈ 0.0015 vs 0.045 mm) gives a smaller friction factor. At the same flow rate, that can shave 20–40% off the head loss compared to steel.