Hazen-Williams Equation Calculator
The water-distribution engineer's everyday equation. Empirical, calibrated for water at typical temperatures, no Reynolds number required.
Defaults: 8-inch ductile iron water main (C = 120), 500 gpm flow, 1000 ft long.
Hazen-Williams C values
- PVC and HDPE (new): 150
- Ductile iron, cement-lined: 140
- Steel pipe (new): 120
- Ductile iron, unlined (older): 100–120
- Cast iron, 30+ years: 80–100
- Tuberculated cast iron: 60–80
- Concrete pipe: 120–140
Distribution-system models commonly use C = 130 for new mains and 100 for older systems. AWWA M14 has a more comprehensive table.
When NOT to use Hazen-Williams
Hazen-Williams is empirical and calibrated for water at roughly 60°F flowing turbulently. It systematically over-predicts head loss in laminar or transitional flow, and is invalid for fluids other than water (oils, slurries, hot water above 100°F or below 40°F). For fire flow (where Re is very high) or any non-water application, use Darcy-Weisbach.
What does C = 120 actually mean?
The C-factor is empirical and not directly traceable to a physical roughness. Higher C means smoother pipe and less friction loss. The exponent 1.852 comes from Hazen and Williams' 1905 fitting of velocity data; it is approximately the Reynolds number exponent for fully rough turbulent flow, which is why the equation works for water-distribution conditions and breaks down outside that range.
Aging
Pipe roughness increases over time as scaling, tuberculation, and biofilm accumulate. C-factor decreases by 10–30 over 20–30 years for unlined ferrous pipe. Cement-lined and plastic pipes are essentially stable for the design life. Use lower C-values when designing for end-of-life capacity.
Reference: AWWA Manual M14 — Design and Construction of Distribution Mains. Original: Williams, G.S., Hazen, A. (1933). Hydraulic Tables (3rd ed.).