Absolute pipe roughness is an effective wall-height input used when estimating friction in a flowing pipe. It has units of length, so it must be compared with the actual inside diameter before it can describe relative roughness. The reference table below reproduces four source-labelled values from a U.S. Geological Survey report. Treat them as a published hydraulic reference set, not as modern product specifications or a measurement of the pipe in front of you.

Pipe internal surface roughness

Absolute roughness and relative roughness

The symbol ε represents absolute roughness. Its units in this table are millimetres. The dimensionless relative roughness is ε divided by the pipe’s inside diameter D, using the same length unit for both. For a 50 mm inside diameter and the USGS steel or wrought-iron entry of 0.0038 mm, ε/D is 0.000076. The 50 mm value is only an example diameter; the table itself does not specify a pipe size.

Relative roughness is used with flow regime information, commonly Reynolds number, to estimate a Darcy friction factor. The U.S. Department of Energy fluid-flow handbook explains that ε/D and Reynolds number are used together with the Moody chart. The table does not directly return pressure drop, capacity, or pump size. A complete calculation also needs fluid properties, flow rate or velocity, actual internal diameter, pipe length, and any minor losses.

How to use this source-specific table

Start by identifying the pipe construction and condition as precisely as the project permits. A material label alone does not tell you whether the wall is clean, lined, scaled, corroded, or affected by joints. Then measure or obtain the inside diameter for the schedule and product series. Use the chosen ε consistently in the selected friction-factor method and retain the source and assumption in the calculation note.

For illustration, the report lists 0.00013 mm for drawn tubing and 0.022 mm for cast iron. At the same inside diameter, the latter produces a much larger ε/D ratio. That comparison shows how a roughness assumption can change the friction model input; it does not mean every cast-iron pipe is 169 times rougher than every drawn tube in service. Real surface condition and the suitability of the cited representative value still matter.

Limits and common mistakes

The source is an older USGS technical report. It preserves a documented set of conventional values, but it does not establish a current condition survey or a manufacturer-certified value for a particular line. Use more specific measurements or project guidance when pipe age, lining, corrosion, or system criticality makes a generic estimate unsuitable. The DOE handbook describes the calculation method; it does not validate these four historic values for a specific installation.

Do not confuse absolute roughness with relative roughness. Do not divide by nominal pipe size when that is different from the actual bore. Do not enter ε as a millimetre tolerance on a drawing, and do not treat a material’s roughness as a surface-finish Ra value. The hydraulic equivalent roughness is a model parameter; it is not a direct profile measurement or a quality limit.

When comparing alternative materials, document both the data source and condition assumed. If a pressure-drop result changes materially across plausible roughness choices, carry the range through the analysis or obtain better data rather than hiding the uncertainty behind one precise-looking number. For commissioning or troubleshooting, measured flow and pressure can be more informative than a roughness estimate alone.

Source and application

The pipe entries are from U.S. Geological Survey Water-Resources Investigations Report 86-4095, Table 2.1. The definition and use of ε/D are described in the DOE Fundamentals Handbook, Volume 3. Verify the appropriate source, pipe condition, units, and fluid model before using any value in a design or operating decision.