Pipe Absolute Roughness (ε) — Darcy-Weisbach Reference
Absolute roughness ε sets the relative roughness ε/D for the Moody diagram and the friction factor f in hf = f·(L/D)·V²/(2g). Values below are for clean, new pipe unless noted. For aging water mains, design to an end-of-life (aged) roughness.
Absolute Roughness by Material
| Material | ε (mm) | ε (ft) |
|---|---|---|
| Drawn tubing, glass, brass, copper | 0.0015 | 0.000005 |
| PVC, HDPE, smooth plastic | 0.0015–0.007 | 0.000005–0.000023 |
| Commercial steel / wrought iron (new) | 0.045 | 0.00015 |
| Asphalted cast iron | 0.12 | 0.0004 |
| Galvanized iron | 0.15 | 0.0005 |
| Ductile iron, cement-mortar lined | 0.10–0.12 | 0.00033–0.0004 |
| Cast iron (uncoated, new) | 0.26 | 0.00085 |
| Wood stave | 0.18–0.9 | 0.0006–0.003 |
| Concrete (smooth to rough) | 0.3–3.0 | 0.001–0.01 |
| Riveted steel | 0.9–9.0 | 0.003–0.03 |
| Corrugated metal pipe (annular) | ~45 | ~0.15 |
| Aged / tuberculated steel or cast iron | 1.0–3.0 | 0.003–0.01 |
Friction Factor Equations (Turbulent, Re > 4000)
Colebrook-White (implicit, the Moody-diagram basis):
Swamee-Jain (explicit, ±1% over 4000 < Re < 108, 10−6 < ε/D < 10−2):
Laminar flow (Re < 2000), roughness irrelevant:
Sources: Moody, L.F. (1944), "Friction Factors for Pipe Flow," Trans. ASME. White, F.M., Fluid Mechanics, Table 6.1. Swamee, P.K. & Jain, A.K. (1976), J. Hydraulics Div., ASCE. Aged values: AWWA M11 / utility practice.
Worked example: new pipe vs. aged pipe
12-in main, V = 4 ft/s, water at 60°F (ν = 1.21×10⁻⁵ ft²/s), 1,000 ft of pipe
This is why water-main design uses an aged roughness for capacity and pressure checks. A system sized on new-pipe values may meet pressure targets on opening day and miss them twenty years later. Plastic and lined pipes hold their roughness much better than unlined iron, which is a large part of their life-cycle advantage.
Common mistakes
- Using ε and D in different units. ε/D is dimensionless only when both are in feet or both in millimeters. A 12-in pipe with ε in feet gives D = 1.0, not 12.
- Using the nominal diameter. Relative roughness and velocity both use the inside diameter, which can differ from nominal by several percent in plastic and lined pipe.
- Applying Colebrook or Swamee-Jain in laminar flow. Below Re ≈ 2,000, f = 64/Re regardless of roughness.
- Converting Hazen-Williams C to ε with a single fixed formula. The two methods disagree in different ways at different velocities and diameters. Pick one method for a calculation and use its own coefficients.
When this card doesn’t apply
The values describe clean pipe with water-like fluids. Slime, scale, sediment and air pockets add resistance that ε does not capture, and tuberculation also reduces the effective diameter. Partly full gravity pipes are normally analyzed with Manning’s n. For HVAC ducts and gas pipelines, use the industry tables for those materials (ASHRAE, AGA), which assume different reference conditions.
Related cheat sheets and tools
Use ε with the Darcy-Weisbach tool, add fitting losses from the minor loss K card, and check the Reynolds number to confirm the flow regime. For water-distribution work the Hazen-Williams equation and its C coefficients are the common alternative. For modeling a full pressurized or stormwater network, see HydroComplete. Fluid properties for these calculations are in the water properties table.