Online Hazen-Williams calculator
How to use the Hazen-Williams calculator
Enter the flow rate in litres per minute (L/min), the pipe's C-factor, internal pipe diameter in millimetres, and the pipe length in metres. The calculator will return the pressure loss per metre (bar/m), the total pressure loss in the pipe (bar), and the water velocity (m/s).
Remember to use the pipe's internal diameter, not the nominal bore — using the nominal diameter will give an incorrect result. If you are unsure which internal diameter to use, refer to the pipe data tables in your design standard or in FHC, which includes a library of 21 standard pipe types.
The common C-factors table below the calculator shows typical values from EN 12845. For a full explanation of how the C-factor affects pressure loss and when different values apply, see our article on the Hazen-Williams equation for fire sprinkler systems.
Hazen-Williams pressure loss
Common C-factors (EN 12845)
About the Hazen-Williams equation
The Hazen-Williams equation is the standard pressure loss formula used in the hydraulic design of fire sprinkler systems worldwide. It is referenced in EN 12845, BS 9251, NFPA 13, and NFPA 750, and uses the pipe roughness coefficient C to account for friction losses without requiring calculation of the Reynolds number.
The equation applies to water flowing in the turbulent range under normal temperature conditions. For systems operating outside these conditions — such as high-pressure water mist systems, or systems using additives like antifreeze or foam — the Darcy-Weisbach equation may be more appropriate. NFPA 750 permits the use of Hazen-Williams for intermediate and high-pressure water mist provided the velocity does not exceed 7.6 m/s and the minimum pipe diameter is at least 20 mm.
Related calculators and tools
If you found this calculator useful, you may also find these tools helpful:
K-factor calculator — calculate flow, pressure, or K-factor for sprinkler heads and water mist nozzles
Pipe velocity calculator — find the velocity in a pipe, or determine the pipe diameter needed for a given flow and velocity
Pressure and flow rate unit converter — convert between bar, psi, kPa, metres head, L/min, L/s, m³/h, and US gal/min
For more complex calculations involving full pipe networks, Hcalc is a free desktop hydraulic calculator you can download and use immediately. For professional fire sprinkler and water mist system design, FHC handles complete hydraulic calculations for tree, loop, and grid networks.
To learn more about the principles behind fire protection hydraulics, visit our hydraulic calculations guide or read the step-by-step walkthrough on how to hydraulic calculate a fire sprinkler system.