Pressure Drop Calculator
Calculate friction loss through pipes using the Hazen-Williams equation. Account for fittings and elevation changes.
About the Hazen-Williams Equation
The Hazen-Williams equation is an empirical formula widely used in plumbing and fire protection to calculate pressure loss due to friction in pipes.
Formula
ΔP = 4.52 × Q^1.85 / (C^1.85 × d^4.87) × L
Where:
ΔP = Pressure drop (PSI)
Q = Flow rate (GPM)
C = Hazen-Williams coefficient
d = Internal pipe diameter (inches)
L = Pipe length (feet)
Where:
ΔP = Pressure drop (PSI)
Q = Flow rate (GPM)
C = Hazen-Williams coefficient
d = Internal pipe diameter (inches)
L = Pipe length (feet)
C Values by Material
- PVC: 150 (smoothest)
- Copper, PEX, CPVC: 140
- Steel, Ductile Iron, Galvanized (new): 120
- Cast Iron: 100
- Galvanized (aged): 80 (rough interior)
Elevation Changes
Elevation change affects pressure at a rate of approximately 0.433 PSI per foot of vertical rise. Going uphill decreases available pressure; going downhill increases it.
Fitting Equivalent Lengths
Fittings add resistance to flow. This calculator uses the equivalent length method, expressing fitting losses as additional pipe length.