PRESSURE LOSS
PRESSURE LOSS
h = 4.52·L·Q^1.852 ÷ (C^1.852·D^4.87)
GPM
ft
RESULT
—
FILL IN ABOVE
Hazen-Williams. Velocity should stay <8 fps cold, <5 fps hot. Old galvanized C-factor drops to 60–80 over decades. Estimate only — verify with a licensed plumber and local plumbing code/inspector before purchase or installation. Not a substitute for engineered drawings.
About this calculator
This pressure loss calculator computes friction loss in a water supply line using the Hazen-Williams equation. Pick the pipe material (different roughness coefficients), nominal size (which determines inside diameter), flow rate, and run length. The output is total head loss in feet, pressure drop in PSI, and water velocity in feet per second — exceeding 8 fps cold or 5 fps hot causes erosion, noise, and water hammer regardless of how the friction math looks. Use this for long service runs, multi-story buildings, irrigation mains, and any branch where the fixture at the dead end is losing pressure. ESTIMATE ONLY — verify with a licensed plumber and local plumbing code before installation.
Common questions
What is the Hazen-Williams formula?
Hazen-Williams calculates friction loss in a water pipe at typical municipal pressures (down to about 60 psi). The formula h = 4.52·L·Q^1.852 ÷ (C^1.852·D^4.87) gives head loss in feet, where L is run length in feet, Q is flow in GPM, C is the pipe roughness coefficient, and D is inside diameter in inches. It's simpler than Darcy-Weisbach (which is more accurate at extreme conditions) and is the industry standard for residential and light commercial water supply.
What is a good C-factor for old galvanized pipe?
New galvanized steel starts at C=120, but mineral buildup and corrosion drop it to 60–90 within 30 years. If you're sizing a retrofit on existing galvanized service, use C=80 for sound 30+ year-old pipe and C=60 if there's visible corrosion or pressure complaints. The math will tell you to upsize, which is usually correct anyway — old galvanized restricts flow even before friction losses are calculated.
Why does velocity matter if my friction loss is acceptable?
Two reasons. Erosion: water above 8 fps (5 fps hot) physically scours the inside of pipe and fittings, especially at elbows, eventually causing pinhole leaks. Water hammer: high velocity means high momentum, so any sudden valve closure produces a pressure spike that can crack joints. Both effects are independent of friction loss — a system can pencil out on PSI drop while still failing on velocity.