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-factor table
The C-factor depends on material and condition. Higher C = smoother = less head loss. For end-of-life capacity analysis on unlined ferrous pipe, drop C by 10–30 from the new-pipe value.
| Pipe material / condition | C (new) | C (aged) |
|---|---|---|
| PVC, schedule 40/80 | 150 | 140 |
| HDPE | 150 | 140 |
| Copper, drawn | 140 | 130 |
| Ductile iron, cement-mortar lined | 140 | 135 |
| Concrete pipe (precast, smooth) | 140 | 120 |
| Steel, riveted, new | 120 | 100 |
| Steel, welded, new | 130 | 110 |
| Steel, galvanized | 120 | 100 |
| Ductile iron, unlined | 120 | 90 |
| Cast iron, unlined, new | 130 | — |
| Cast iron, 10 years old | — | 110 |
| Cast iron, 30 years old | — | 90 |
| Cast iron, tuberculated (40+ yr) | — | 60–80 |
| Asbestos cement | 140 | 120 |
| Vitrified clay (sewer) | 110 | 100 |
Source: AWWA Manual M14 — Recommended Practice for Backflow Prevention and Cross-Connection Control; AWWA M11 — Steel Pipe. Distribution-system models commonly default to C = 130 for new mains and C = 100 for older systems.
Worked examples
Example 1 — 8-inch ductile iron water main, 1500 ft, 600 gpm
Example 2 — Sizing for fire flow (1500 gpm, max hf/L = 0.01)
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.).