Reference
Absolute roughness ε feeds the relative roughness ε/D in the Colebrook–White equation and the Moody chart. It matters most in fully turbulent flow, where the friction factor becomes independent of Reynolds number and depends only on ε/D. Values below are typical for clean, new pipe; ageing, scaling, and corrosion increase roughness substantially.
| Material | ε (mm) | ε (ft) |
|---|---|---|
| Drawn tubing (copper, brass, glass) | 0.0015 | 5.0e-6 |
| PVC / plastic | 0.0015 | 5.0e-6 |
| Stainless steel | 0.015 | 4.9e-5 |
| Commercial / wrought steel (new) | 0.045 | 1.5e-4 |
| Asphalted cast iron | 0.12 | 3.9e-4 |
| Galvanized iron | 0.15 | 4.9e-4 |
| Cast iron | 0.26 | 8.5e-4 |
| Wood stave | 0.18–0.9 | 6e-4–3e-3 |
| Concrete | 0.3–3.0 | 1e-3–1e-2 |
| Riveted steel | 0.9–9.0 | 3e-3–3e-2 |
Clean, new-pipe values. Corrosion and scale can raise ε by an order of magnitude over a pipe's life.
Absolute roughness ε is the average height of the surface irregularities, in length units. Relative roughness is ε divided by the pipe inside diameter (ε/D), the dimensionless quantity that actually enters the Colebrook equation and the Moody chart.
New commercial or wrought steel is about 0.045 mm. If the line is old, corroded, or scaled, use a larger value — roughness can increase tenfold or more over time, and pressure drop rises with it.