Pipe Pressure Drop Calculator (Water)
Hazen-Williams head loss and pressure drop for water flowing through any pipe size, material, and length. Also outputs velocity.
Total pressure drop
—
— bar
Head loss
—
ft of water
Per 100 ft
—
psi/100 ft
Velocity
—
ft/s
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About the Hazen-Williams equation
Hazen-Williams is the workhorse formula for water pressure drop in pipes. It's an empirical equation (curve-fit to test data) limited to water at typical building temperatures, but for that case it's accurate enough for almost any sizing decision in plumbing, fire protection, and irrigation work. For oils, hot water above ~140°F, or compressible gases, use Darcy-Weisbach instead.
hf (ft) = 4.52 × L × Q^1.852 / (C^1.852 × D^4.87)
ΔP (psi) = hf × 0.4331 (water at 60°F)
Velocity (ft/s) = 0.4085 × Q / D² (Q in GPM, D in inches)
C-factor (roughness coefficient)
- 150 — PVC, CPVC, smooth plastic, new copper
- 140 — typical copper
- 130 — new carbon steel, cement-lined ductile iron
- 120 — steel a few years old, galvanized
- 100 — old steel with light corrosion
- 80 — old cast iron, heavy scale
C-factor drops over time as pipe interior roughens. Design conservatively — if you're sizing for a 20-year service life, use C = 100 for steel even if the pipe is brand new.
Velocity guidance
- Cold water service: 5–8 ft/s typical, 10 ft/s max
- Hot water service: 4–5 ft/s to limit erosion-corrosion
- Fire protection: typically sized to keep loss reasonable, not by velocity
- Slurries, particulate flow: 4–6 ft/s to prevent settling
- Suction lines (pump inlets): 2–4 ft/s to limit NPSH loss
Frequently asked questions
What is the Hazen-Williams equation used for?
Hazen-Williams is an empirical formula for calculating head loss (pressure drop) in pipes carrying water. It's the standard tool for sizing plumbing distribution systems, fire sprinkler systems, and irrigation mains. It's limited to water at typical building temperatures (40–75°F) — for hot water above 140°F or other fluids, use the Darcy-Weisbach equation instead.
What C-factor should I use for my pipe?
Use C = 150 for new PVC or copper, C = 140 for typical in-service copper, C = 130 for new carbon steel, C = 120 for galvanized or aged steel, and C = 100 for old steel with light corrosion. When designing for long service life, use a lower C than the pipe's current condition — a new steel pipe might start at 130 but be at 100 after 20 years of service. Conservative C values protect against under-sizing.
What water velocity is acceptable in a pipe?
For cold water service, 4–8 ft/s is the typical design range, with 10 ft/s as a practical maximum. Hot water service should stay below 5 ft/s to limit erosion-corrosion at fittings. Pump suction lines should stay at 2–4 ft/s to preserve NPSH margin. High velocity causes noise, erosion, and water hammer — if the calculator shows over 8 ft/s, consider going up one pipe size.
How do I convert head loss (feet of water) to psi?
Multiply feet of head by 0.4331 to get psi (for water at 60°F). Going the other way: divide psi by 0.4331 to get feet of head. This comes from water density — 1 psi = 2.308 ft of water column. The calculator displays both automatically. For fluids other than water, multiply the fluid's specific gravity into the conversion.
How does pipe schedule affect pressure drop?
Schedule determines wall thickness, which changes the inside diameter (ID). A larger ID means lower velocity and dramatically lower friction loss — pressure drop scales with D to the 4.87 power in the Hazen-Williams equation. For example, going from Schedule 80 to Schedule 40 on a 2" pipe increases the ID from 1.939" to 2.067", reducing pressure drop by about 25% at the same flow rate.