Pipe roughness values
Absolute roughness of common pipe materials, used to find the friction factor for pressure-drop calculations. Roughness is the average height of the surface irregularities on the inside wall, and it sets how much energy the flow loses to friction.
Absolute roughness
| Material | Roughness (mm) | Roughness (ft) |
|---|---|---|
| Drawn tubing (copper, glass) | 0.0015 | 0.000005 |
| PVC and plastic | 0.0015 | 0.000005 |
| Stainless steel | 0.015 | 0.00005 |
| Commercial steel | 0.045 | 0.00015 |
| Wrought iron | 0.046 | 0.00015 |
| Asphalted cast iron | 0.12 | 0.0004 |
| Galvanized steel | 0.15 | 0.0005 |
| Cast iron | 0.26 | 0.00085 |
| Concrete | 0.3 to 3.0 | 0.001 to 0.01 |
| Riveted steel | 0.9 to 9.0 | 0.003 to 0.03 |
Absolute roughness, written as the Greek letter epsilon, is divided by the pipe diameter to give the relative roughness. That, together with the Reynolds number, is read off a Moody chart (or solved with the Colebrook equation) to find the friction factor, which then sets the pressure drop. Smoother pipe means lower friction and less pumping energy.
Calculating flow regime or pressure drop?
See the Reynolds Number Flow Regimes chart, or the Reynolds Number Calculator.
How roughness drives friction
In turbulent flow the friction factor depends on the relative roughness, the ratio of wall roughness to pipe diameter. Rougher walls trip up the flow near the surface, raising friction and pressure loss. In fully rough flow the friction factor depends almost entirely on relative roughness, which is why material choice matters so much for long pipelines.
New vs aged pipe
Published roughness values are for clean, new pipe. Over years of service, scale, corrosion, and deposits can multiply the effective roughness several times, raising friction and cutting flow. Designers often add a margin to the roughness, or to the pump head, to keep performance acceptable as the pipe ages.
FAQ
What is the roughness of commercial steel pipe?
About 0.045 mm (0.00015 ft) when new. It is the standard reference value for ordinary steel piping.
Why does pipe roughness matter?
It sets the friction factor, and so the pressure drop and pumping power. Rougher pipe loses more energy to friction for the same flow.
Does smooth plastic pipe have less friction?
Yes. Drawn and plastic pipe have very low roughness near 0.0015 mm, giving lower friction than steel or cast iron of the same size.