The Hazen-Williams coefficient C appears in an empirical equation used to estimate head loss in water pipelines. It summarizes hydraulic resistance within that equation; it is not a direct measurement of wall roughness and is not interchangeable with a Darcy friction factor. The selected values here are from a U.S. Environmental Protection Agency SWMM hydraulics reference table. They are generic modeling inputs, not certified values for every pipe sold under the same material name.

Pipe Flow Coefficients by Material

Read the table as a reference

The EPA table lists C=140 for asbestos cement and ductile iron, C=130 for PVC, and C=120 for galvanized iron and concrete. These labels and numbers should travel together with the source context. The Ductile Iron Pipe Research Association also describes C=140 as a design value for waterworks based on field tests. That does not turn the number into a universal constant; it emphasizes the need to use a value appropriate to the system, pipe condition, and design authority.

Use the right equation and units

A common SI form relates friction head loss to length, flow, C, and inside diameter. The diameter term has a strong exponent, so use the actual hydraulic inside diameter rather than nominal size. Keep flow, length, and diameter units consistent with the equation constant. If converting the equation to customary units, use the matching coefficient and unit definitions. A spreadsheet that mixes metres and feet or uses nominal diameter without checking the bore can produce a plausible-looking but materially wrong result.

Material alone does not set performance

Age, corrosion, scale, deposits, lining, joints, and installation history can change effective hydraulic behavior. So can the actual pipe series and inside diameter. A smooth new plastic pipe and an aged or damaged line cannot be assumed to share the same field performance solely because a broad material label matches. Designers may use different C values for initial versus long-term conditions. Document why the selected coefficient fits the expected service life and whether the design uses a conservative allowance.

Know when the method applies

Hazen-Williams is commonly used for water-system calculations but is empirical and has defined application limits. It is not a general gas-flow equation, and the coefficient is not a substitute for a Reynolds-number-based Darcy-Weisbach analysis when that method is required. Do not apply these tabulated values to a different fluid or claim universal validity over all temperatures and regimes. For systems where viscosity, compressibility, or unusual operating conditions matter, choose the governing hydraulic model and verify it with appropriate engineering data.

A practical selection workflow

First identify the fluid and governing design method. Confirm actual inside diameter, pipe age, lining, and expected service condition. Select a coefficient from a source that matches the application, then record the publication and any design assumptions. Calculate head loss and review sensitivity to plausible changes in C and diameter. Compare the result against pressure requirements, pump duty, and applicable local criteria. For a critical water supply or process line, obtain project-specific guidance and check results with a qualified engineer rather than relying on this compact comparison alone. When comparing alternatives, hold the flow rate and internal diameter fixed so the coefficient effect is visible. Then rerun the calculation with the proposed pipe dimensions and operating range; otherwise a material comparison can hide differences caused by bore size or duty point.

Sources and verification

Values are a selected subset of U.S. EPA SWMM Reference Manual, Volume II: Hydraulics, Table 7-3. Application context for ductile iron is discussed in the DIPRA FAQ.

This table is a reference aid, not a substitute for the governing design code, product specification, qualified engineering review, or measurements from the actual component and service conditions.