Friction Loss Calculator
When water flows through a pipe, friction between the water and the pipe wall causes pressure to drop. This calculator uses the Hazen-Williams equation to find the friction loss for any pipe run. A 100 GPM flow through 100 feet of 2-inch PVC pipe loses about 5.9 PSI, for example. Firefighters, plumbers, irrigation designers, and HVAC engineers all use this calculation.
The Hazen-Williams formula:
hf = (4.52 × Q^1.852) / (C^1.852 × D^4.8655)
Where hf is head loss per foot of pipe (in feet of water), Q is flow in GPM, C is the roughness coefficient, and D is the inside pipe diameter in inches.
The C factor explained:
The C factor represents how smooth the pipe interior is. Higher values mean less friction:
- PVC/CPVC (150): Very smooth, lowest friction loss
- New copper (140): Smooth metal surface
- Cement-lined ductile iron (130): Common in municipal water mains
- New cast iron (120): Slightly rougher than coated pipe
- Old cast iron (100): Decades of corrosion and mineral buildup
- Severely corroded (80): Old, unlined pipes in poor condition
Flow velocity guidelines:
The velocity output helps you check whether the flow rate is appropriate for the pipe size. Recommended maximum velocities:
- Domestic plumbing: 5-8 ft/s
- Fire sprinkler mains: up to 20 ft/s (during flow)
- Municipal water mains: 3-5 ft/s
Velocities above 10 ft/s cause noise, erosion, and water hammer. If the calculator shows a high velocity, consider upsizing the pipe.
Firefighting application:
Fire departments use a simplified friction loss formula for hose: FL = CQ²L, where C is a hose coefficient, Q is flow in hundreds of GPM, and L is hose length in 100-foot sections. This pipe calculator is more accurate for fixed piping systems.