Hazen-Williams Head Loss Calculator

What is the Hazen-Williams equation?

Hazen-Williams is an empirical equation that gives friction loss in pressurised water pipes without working out a Reynolds number or friction factor. In SI units it reads v = 0.849 × C × R^0.63 × S^0.54, where v is the mean velocity, R the hydraulic radius (d / 4 for a full circular pipe), S the head loss per unit length and C a factor that depends on the pipe wall. The working form of the same equation is h = 10.67 × L × Q^1.852 / (C^1.852 × d^4.87).

The C factor

A higher C means a smoother pipe and less loss. Plastic pipe sits around 140 to 150, new cast iron near 130 and galvanized steel near 120. Aged and scaled pipe can fall below 100. These figures are approximate, so for design enter the value given by the standard or the manufacturer.

Example

Water flows at 30 litres per second through 500 metres of pipe with a 150 mm bore and C = 130. The velocity is 1.70 m/s, the loss is 20.2 metres per kilometre, and the total head loss is 10.1 metres of water column, about 0.99 bar.

Limits

The equation applies only to water at ordinary temperatures in turbulent flow. It becomes unreliable for bores under 50 mm and velocities above 3 m/s. For other fluids, hot water or precise work use the Darcy-Weisbach equation. Minor losses and elevation change are not included.