Friction Loss Calculator
This tool uses the Hazen-Williams equation, the industry-standard empirical formula for friction (major) losses of water flowing full in a pressurised pipe.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the design flow rate in litres per minute and the pipe's INTERNAL diameter in millimetres (use the bore, not the nominal or outside diameter).
- Enter the run length in metres and the Hazen-Williams C factor for the pipe material (about 150 for new PVC/PE, 130-140 for copper, 100 for older galvanised steel).
- Read the friction head loss, the equivalent pressure loss in kPa, the loss per metre, and the mean velocity, then compare against your pump head budget and local velocity limits.
How it works
This tool uses the Hazen-Williams equation, the industry-standard empirical formula for friction (major) losses of water flowing full in a pressurised pipe. In SI form the head loss is hL = 10.67 × L × Q^1.852 ÷ (C^1.852 × D^4.87), where hL is metres of water, L is length in metres, Q is flow in m³/s, D is internal diameter in metres, and C is the roughness coefficient of the pipe material. The calculator converts your L/min and mm inputs to these base units first, then reports the result back as metres of head, kilopascals (1 m of water ≈ 9.80665 kPa), millimetres of loss per metre, and the mean velocity V = Q ÷ (π·D²/4).
The higher the C factor the smoother the pipe and the lower the loss, so a smooth new plastic pipe (C ≈ 150) loses far less head than corroded steel (C ≈ 100). Note that Hazen-Williams covers only straight-pipe friction — it does not include minor losses from bends, tees, valves and fittings, which are added separately using equivalent lengths or K factors. It is valid for water near ordinary temperatures and typical plumbing velocities, and is less accurate for very hot water, other liquids, or very low flows where the Darcy-Weisbach method is preferred.
Worked example
60 L/min through 50 m of 25 mm PVC (C = 150). Enter Flow 60 L/min, Diameter 25 mm, Length 50 m and C factor 150. The calculator converts to SI (Q = 0.001 m³/s, D = 0.025 m) and applies Hazen-Williams to return about 5.17 m of friction head loss, roughly 50.66 kPa of pressure loss, 103.34 mm/m along the pipe, and a mean velocity near 2.04 m/s. That velocity is within typical limits, so the note confirms the pipe is acceptable for that duty.
Common mistakes
- Entering the nominal or outside pipe diameter instead of the true internal bore — friction loss scales with D^4.87, so a small diameter error causes a large error in the result.
- Forgetting that the result is straight-pipe friction only; real installations must add minor losses for every bend, tee and valve, plus any static lift, before sizing a pump.
- Using the wrong C factor for the material or its age — new PVC/PE is about 150, but old galvanised or tuberculated steel can drop to 100 or lower, roughly doubling the loss.
Frequently asked questions
What is a good Hazen-Williams C factor to use?
Use about 150 for new PVC, PEX and polyethylene, 130-140 for copper and smooth new steel, 120 for cement-lined ductile iron, and 100 or lower for older galvanised or corroded steel. When in doubt use a conservative (lower) value, and always confirm against the manufacturer's data and your local code.
Does this include losses from bends, valves and fittings?
No. This calculates friction (major) loss for straight pipe only. Fitting losses (minor losses) are added separately using equivalent pipe lengths or K-factor coefficients for each bend, tee and valve, then summed with the straight-pipe result.
How do I convert the head loss to pressure?
One metre of water head equals about 9.80665 kPa (roughly 1.42 psi). The tool shows the kPa value automatically. Add this friction loss to any static lift and fitting losses to find the total head your pump must overcome.
Should I use Hazen-Williams or Darcy-Weisbach?
Hazen-Williams is quick and standard for cold water reticulation and plumbing, which is what this tool targets. Darcy-Weisbach with the Moody chart is more general and preferred for hot water, other fluids, wide temperature ranges, or unusual velocities. For code-compliant designs, verify against your local plumbing/building code and have a licensed professional review the result.
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Tip: Enter any known values to calculate the remaining results.
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