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.

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