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Pipe Friction Loss Calculator

Calculate friction head loss in a water pipe using the Hazen-Williams equation — essential for pump and pipeline sizing.

Friction head loss (m) = 10.67 × L × Q¹·⁸⁵² ÷ (C¹·⁸⁵² × D⁴·⁸⁷), where Q is in m³/s and D is in m. A 200 m run of 100 mm PVC pipe at 5 L/s loses about 5.6 m of head.

Calculator

Friction head loss
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How it works

The Hazen-Williams equation is the standard method for friction loss in water-supply pipes. Head loss increases steeply with flow rate and drops sharply with pipe diameter — doubling the diameter reduces friction loss by roughly 25×. The C factor captures pipe roughness: higher C means a smoother pipe and less friction.

Worked example

200 m of 100 mm PVC (C = 150) at 5 L/s: Q = 0.005 m³/s, D = 0.1 m. hf = 10.67 × 200 × (0.005)^1.852 ÷ (150^1.852 × 0.1^4.87) ≈ 5.6 m. Add this to the static lift to get total dynamic head (TDH) for pump selection.

Assumptions & limitations

Hazen-Williams is accurate for turbulent flow in water-supply pipes (Reynolds number 10⁴–10⁶, velocity 0.5–3 m/s). It is not suitable for viscous fluids or very low/high velocities. This result covers straight-pipe friction only — add minor losses (bends, valves, fittings) separately, or add 10–15% as a quick allowance for simple systems.

Frequently asked questions

What is the Hazen-Williams C factor?

C is a roughness coefficient indicating how smooth the pipe interior is. Higher C = smoother = less friction loss. New PVC and HDPE are C 140–150; galvanised steel is C 120; aged cast iron with scale deposits may drop to C 80–100. Use the manufacturer's value for new pipe and reduce it for systems that have been in service for years.

How do I use this result to size a pump?

Total Dynamic Head (TDH) = static head (elevation difference between source and discharge) + friction head loss + minor losses. Add the friction head from this calculator to your static head, then select a pump whose performance curve meets your required flow at that TDH.

What flow velocity is acceptable in a pipe?

For water supply, keep velocity below 1.5–2.0 m/s to limit friction loss and water-hammer risk. Velocity (m/s) = flow rate (m³/s) ÷ (π × r²). For 5 L/s in a 100 mm pipe: v = 0.005 ÷ (π × 0.05²) = 0.64 m/s — well within the safe range.

How do I account for bends, valves, and fittings?

Use the equivalent-length method: each fitting has an equivalent pipe length (a 90° elbow ≈ 30 × pipe diameter; a gate valve ≈ 7 × diameter). Add these to the actual pipe length before running the calculation. For a quick estimate on simple layouts, adding 10–15% to the friction loss result is usually sufficient.

Last updated 22 June 2026

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