Pipe Roughness Calculator
Every pipe material has a different surface texture - and that roughness directly controls how much friction the flowing fluid experiences. Select your pipe material to get its absolute roughness (ε), then calculate the relative roughness (ε/D) needed for the Moody Chart Calculator.
Calculate Relative Roughness (ε/D)
Results
What Is Pipe Roughness and Why Does It Matter?
Run your finger across the inside of a new PVC pipe - it's almost glassy smooth. Now imagine the inside of an old cast iron main - it's covered with pits, lumps, and scale. That surface texture is what engineers call absolute roughness (ε), measured in millimeters or micrometers.
In turbulent flow, the rough surface creates tiny vortices near the pipe wall that dramatically increase friction compared to a smooth pipe. The rougher the pipe - relative to its size - the higher the friction factor, and the more energy the pump must supply. This is why the Moody Chart has a whole family of curves, one for each level of relative roughness.
Absolute vs Relative Roughness
Absolute roughness (ε) is a property of the pipe material - the average height of surface irregularities. A 100 mm steel pipe and a 500 mm steel pipe have the same absolute roughness: ε = 0.046 mm.
Relative roughness (ε/D) is what actually matters for friction. That same 0.046 mm roughness is significant in a 10 mm pipe (ε/D = 0.0046) but almost negligible in a 1000 mm pipe (ε/D = 0.000046). Relative roughness is what you plot on the Moody Chart.
Roughness Values by Material
How Roughness Changes Over Time
New pipe roughness values are a starting point. Over years of service, corrosion and scale buildup can increase effective roughness by 3–10× in steel and iron pipes. The Hazen-Williams C-factor method tries to account for this empirically, but if you're using the Darcy-Weisbach equation (which is more accurate), apply an aging factor to your roughness value.
For more on pipe roughness and its effects, read our Pipe Roughness: A Practical Guide.
Frequently Asked Questions
My pipe material isn't listed. What roughness should I use?
Use the "Custom" option and enter a value. If you don't know the roughness, 0.046 mm (commercial steel) is a conservative default for metal pipes. For smooth plastic or lined pipes, use 0.0015 mm.
Does roughness matter in laminar flow?
No. In laminar flow (Re < 2300), friction factor depends only on Reynolds number: f = 64/Re. Roughness has no effect. Only in turbulent flow does roughness control friction - this is why the Moody Chart curves spread out at high Reynolds numbers.
What is the hydraulically smooth regime on the Moody Chart?
When the roughness elements are small enough to be submerged within the viscous sublayer near the pipe wall, the pipe behaves as if it were perfectly smooth. This happens at low Reynolds numbers in turbulent flow. At very high Re, the sublayer disappears and roughness fully controls friction - the "fully rough" regime where friction factor is independent of Re.
Related Tools
📊 Moody Chart Calculator
Enter ε/D from this tool and Reynolds number to find friction factor.
📏 Relative Roughness Calculator
Simple ε/D calculation if you already know the absolute roughness.
🌊 Reynolds Number Calculator
The other Moody Chart input - determine flow regime first.
📉 Pressure Drop Calculator
Use friction factor from the Moody Chart to find pressure drop.