How do you calculate relative roughness?

Relative roughness is calculated by dividing the absolute surface roughness (ε) by the internal pipe diameter (D). Formula: Relative Roughness = ε / D.

Surface roughness of common materials

Q: What is the surface roughness of a steel pipe?

The absolute roughness of a new commercial steel pipe is typically around 0.05 mm (0.002 inches). If the pipe is rusted, the roughness can increase to 0.15-0.25 mm.

Pipe roughness for friction factor and pressure drop calculation

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Section summary
1. Surface roughness of common materials
2. Relative roughness
3. Interactive Roughness Calculator

1. Surface roughness of common materials

What is the surface roughness of a steel pipe? What is the surface roughness of concrete? The roughness is actually a measurement of all the asperities at the surface of a material, especially a pipe where it will oppose the flow of fluid.

⚠️ ENGINEERING NOTICE & EDUCATIONAL DISCLAIMER: This interactive calculator is provided exclusively for preliminary estimation and educational purposes. It is not intended for detailed design or equipment procurement without certified vendor rating.

Pipe Relative Roughness Calculator

Material Preset

Absolute Roughness (\(\epsilon\)) [mm]

Internal Diameter (\(D\)) [mm]

Relative Roughness (\(\epsilon/D\)): -

Roughness Class: -

Industrial Best Practices & Rules of Thumb

Standard Steel: For new commercial steel pipes, \(\epsilon = 0.05\) mm is the standard engineering assumption.
Aging Allowance: Always consider a "fouled" or "rusted" roughness for long-term hydraulic calculations. Roughness can increase by 5-10x over 20 years in corrosive services.
Critical Limit: If \(\epsilon/D > 0.05\), the pipe is considered "extremely rough." Pressure drop correlations like Colebrook-White may be less accurate.
Plastic Advantage: PVC and HDPE (\(\epsilon \approx 0.005\) mm) offer significantly lower pressure drops than steel, allowing for energy savings or smaller diameters.

The following table summarizes usual pipe roughness for many materials used for piping or ducts:

Material	Absolute Roughness (mm)
Steel pipe - stretched	0.015
Steel pipe - commercial, laminated - NEW	0.05
Steel pipe - commercial, laminated - RUSTED	0.15-0.25
Steel pipe - welded NEW	0.03-0.1
Steel pipe - welded RUSTED	0.4
Iron pipe - galvanized	0.15-0.20
Cast iron - molded NEW	0.25
Cast iron - molded RUSTED	1-1.5
Glass, copper or brass stretched pipe	0.001
Concrete pipe - smoothed	0.3-0.8
Concrete pipe - coarse	3
Plastic pipe	0.002-0.007
Wood - ordinary	1-2.5
Stone (cut)	8-15

For steel pipes, when nothing more is specified, 0.05 mm is a good first approximation.

2. Relative roughness

The relative roughness is calculated by dividing the absolute pipe roughness (\(\epsilon\)) over the pipe internal diameter (\(D\)). Both values must be in the same units.

\[ \text{Relative Pipe Roughness} = \frac{\epsilon}{D} \]

Relative Pipe Roughness = ε / D

In a more general expression for non-circular ducts, the diameter can be replaced by the hydraulic diameter (\(D_h\)).

What is the pipe roughness used for? The pipe relative roughness is used to calculate the friction factor (using correlations like Colebrook-White or Churchill), which is then used to determine the pressure drop for a fluid flow. You can visualize these relationships on the Moody chart.

Source:
Mecanique et Rheologie des fluides en genie chimique, Midoux, Lavoisier Tec et Doc, page 277