How it’s calculated
- Friction (psi) = 4.52 × L × Q^1.852 ÷ (C^1.852 × d^4.8704)
- Q in gpm, d inside diameter in inches, L in feet
- Elevation (psi) = 0.433 × rise (ft)
Worked example
10 gpm through 100 ft of 1 in type L copper (1.025 in ID), level run.
Inputs
- Flow rate
- 10 gpm
- Pipe inside diameter
- 1.025 in
- Pipe length (including fitting allowance)
- 100 ft
- Pipe material (roughness)
- Copper (C = 140)
- Elevation rise (negative for drop)
- 0 ft
Results
- Total pressure loss
- 3.02 psi
- Friction loss
- 3.02 psi
- Elevation loss
- 0 psi
- Velocity
- 3.89 ft/s
Assumptions and limits
- Hazen-Williams applies to water near room temperature in turbulent flow. It is less accurate for hot water, very small pipes and very low velocities.
Sources: Hazen-Williams equation, US customary form (ASPE/Cameron Hydraulic Data).
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
How much pressure do I lose per floor?
About 0.433 psi per foot of height, so a 10 ft floor-to-floor rise costs about 4.3 psi before friction.
What C value should I use for PEX?
Smooth plastics are commonly modelled with C = 150. Fittings in PEX systems have significant restriction — add equivalent length for them.