Hazen-Williams Pipe Flow Calculator

The Hazen-Williams pipe flow calculator estimates the friction head loss in a full, pressurised water pipe using the popular Hazen-Williams empirical equation in SI form. It is the go-to method for water supply and irrigation work because it needs only one roughness coefficient and no fluid viscosity. You enter four inputs: the flow rate Q in cubic metres per second, the pipe length L in metres, the internal diameter d in metres, and the Hazen-Williams roughness coefficient C. The calculator returns the head loss in metres of water across that length of pipe. Hydraulic engineers, plumbers, irrigation designers and councils use it to size mains, check pump duty, and confirm that pressure at the far end of a network is adequate. The coefficient C describes pipe smoothness: roughly 150 for new plastic or PVC, 130 to 140 for new cement-lined ductile iron, around 100 for older steel, and lower again for tuberculated or aged pipes. Picking the right C is the single biggest influence on accuracy. A few tips help. First, use the actual internal diameter, since nominal pipe sizes can differ noticeably from the bore and diameter is raised to a high power. Second, remember Hazen-Williams is calibrated for water near normal temperatures and ordinary turbulent flow, so it is not suited to other fluids or very low velocities. Third, add separate allowances for fittings, valves and bends, because this equation covers straight-pipe friction only. Always cross-check critical designs against a recognised hydraulic standard.

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1.90 m
Head loss
Per 100 m of pipe1.90 m

hf = 10.67 x L x Q^1.852 / (C^1.852 x d^4.87), SI units. Estimate only, not engineering design advice.

How it works

Head loss equals 10.67 times the length times the flow rate raised to the power 1.852, divided by the coefficient C raised to 1.852 and the diameter raised to 4.87. Higher flow or rougher, narrower pipe increases the loss, with diameter having the strongest effect.

Worked example

With Q of 0.01 m3/s, length 100 m, diameter 0.1 m and C of 130, the numerator is 10.67 times 100 times 0.01^1.852. Dividing by 130^1.852 times 0.1^4.87 gives a head loss of about 1.90 m over the 100 m run.

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Frequently asked questions

What is the C coefficient?

C is the Hazen-Williams roughness factor describing how smooth the pipe bore is. Higher values mean smoother pipe and lower friction loss. Typical figures are around 150 for new plastic, 130 to 140 for new lined iron and about 100 or less for old steel.

Does Hazen-Williams work for any fluid?

No. It is an empirical equation calibrated specifically for water at ordinary temperatures in turbulent flow. For other liquids, gases or unusual conditions you should use the Darcy-Weisbach equation instead, which accounts for viscosity and the flow regime.

Does this include losses from fittings?

No, it gives the friction loss along a length of straight pipe only. Bends, valves, tees and fittings add extra minor losses that you must calculate and add separately. Ignoring them can understate the total head loss in a real network.