Hazen-Williams C — Pipe Roughness Reference
For pressurized water-distribution head loss: hf = 4.73·L·Q1.852 / (C1.852·D4.87) (US units, hf in ft, L & D in ft, Q in cfs). Use this for water at ~60°F in transmission and distribution mains. For other fluids, low-flow regimes, or where temperature matters, use Darcy-Weisbach instead.
Plastic Pipe
| Material | C (new) | C (design) | Notes |
|---|---|---|---|
| PVC, smooth bore | 150 | 140–150 | AWWA C900/C905 |
| HDPE, DR 11–17 | 150 | 140–150 | AWWA C906; very stable with age |
| Polyethylene, small dia. | 140 | 130–140 | service lines |
Iron & Steel Pipe
| Material | C (new) | C (design / aged) | Notes |
|---|---|---|---|
| Ductile iron, cement-mortar lined | 140 | 130–140 | AWWA C151/C104; design value typically 130 |
| Ductile iron, unlined (rare) | 130 | 90–110 | tuberculation reduces C dramatically |
| Cast iron, new | 130 | 100–130 | |
| Cast iron, 20+ yr unlined | 100 | 60–100 | severe tuberculation possible |
| Steel, cement-mortar lined | 140 | 130–140 | AWWA C200/C205 |
| Steel, coal-tar enamel lined | 140 | 130–140 | AWWA C203 |
| Steel, riveted (legacy) | 110 | 90–110 | |
| Galvanized steel | 120 | 100–120 |
Concrete & Other
| Material | C (new) | C (design / aged) | Notes |
|---|---|---|---|
| Concrete, smooth-formed | 130 | 120–140 | |
| Concrete pressure pipe (PCCP) | 140 | 130–140 | AWWA C301/C303 |
| Asbestos-cement (legacy) | 140 | 130–140 | still in service in older systems |
| Copper tubing | 140 | 130–140 | building service |
| Brass | 130 | 120–130 | |
| Vitrified clay (sewer, full flow) | 110 | 100–120 |
When to use Hazen-Williams vs. Darcy-Weisbach
| Use Hazen-Williams when… | Use Darcy-Weisbach when… |
|---|---|
| Water at ~50–80°F | Other fluids, hot/cold extremes, viscous flow |
| Turbulent flow (typical mains) | Laminar / transitional flow (Re < 4000) |
| Fully-developed water-distribution pipes | Compressible flow, two-phase, complex networks needing energy balance |
| You need a quick spreadsheet calc | You need one method across fluids or temperatures, or your model is set up for Darcy-Weisbach (EPANET offers H-W, D-W and Chezy-Manning) |
Sources: AWWA M11 (steel), M22 (sizing water service), M23 (PVC), M55 (HDPE). Mays, L.W. (2010). Water Distribution Systems Handbook. Hwang & Houghtalen, Fundamentals of Hydraulic Engineering Systems.
Worked example: what aging costs a main
12-in main, 1,000 ft, 1,500 gpm (3.34 cfs, V = 4.26 ft/s)
Because C is raised to the 1.852 power, a modest change in C moves head loss a lot. Choosing C is a design decision, and it should be documented as one. For new mains, most utilities specify the C to use. For existing mains, a fire-flow or C-factor field test gives the real value, and older unlined mains often test far below what their material would suggest.
Common mistakes
- Mixing unit systems. The 4.73 coefficient takes Q in cfs and L and D in feet. The common gpm-and-inches form uses 10.44, and the SI form uses 10.67. Using the wrong constant for your units gives a wrong answer that may still look plausible.
- Using nominal diameter. Head loss scales with D4.87, so a 2% error in diameter becomes roughly a 10% error in head loss. Use the actual inside diameter for the pipe class.
- Applying Hazen-Williams to hot water, other fluids or very small pipe. The formula has no viscosity term and was fitted to water at ordinary temperatures in larger pipes.
- Using the new-pipe C for a 50-year design. See the design tip above.
When this card doesn’t apply
Hazen-Williams is empirical, and it loses accuracy at very low and very high velocities and outside water-distribution sizes. For hydraulic models, EPANET lets you choose Hazen-Williams, Darcy-Weisbach or Chezy-Manning headloss, and Hazen-Williams is its default. Use the same formula and coefficients across a model. Sewage force mains with grease and solids, slurries, and fire-sprinkler systems governed by NFPA 13 each have their own required methods and C values.
Related cheat sheets and tools
Designing a transmission main? The companion to this table is the Hazen-Williams equation reference — every form, every unit constant, and the validity limits. You'll also want Reynolds number to confirm turbulent regime, and Manning's n if any of the pipe runs partial-flow as gravity sewer. For full water-system network analysis with multiple loops and pressure zones, EPANET is free and uses Darcy-Weisbach internally. For full stormwater modeling that includes pressurized conduits, see HydroComplete — the SaaS sister product to PE-Calc.