Hazen-Williams C Coefficients for New Pipe

The Hazen-Williams coefficient C is an empirical factor used in one common method for estimating water flow and head loss in full pipes. The selected values below are EPA EPANET 2.2 reference coefficients for new pipe. They are unitless and are not interchangeable with Darcy-Weisbach absolute roughness or Manning’s n.

Selected coefficients for new pipe

EPANET lists ranges for several materials rather than one universal number. The table keeps the source’s new-pipe scope visible; it does not describe every lining, diameter, installation, water condition, or service age.

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What the C value represents

In the Hazen-Williams relation, a larger C generally represents less friction loss for the same modeled flow and pipe dimensions. It is a practical calibration coefficient, not a direct measurement of inside-surface roughness. Pipe age, corrosion, deposits, lining, joints, and local operating history can change the appropriate value. Do not interpret the table as a material warranty or assume that a new-pipe value remains constant over the life of a system.

EPA’s manual also presents Darcy-Weisbach and Manning approaches. Each uses a different resistance parameter and formulation. The numbers are not conversions of one another: an EPANET Hazen-Williams C value cannot be entered as Darcy roughness or Manning’s n. Match the equation, units, and coefficient to the method used by the analysis software or design guidance.

Keep the method in its proper range

Hazen-Williams is an empirical water-flow method, commonly used in water-distribution calculations. It is not a general gas-flow equation and does not account for compressibility. Do not use these coefficients to size natural-gas, compressed-air, steam, or other gas lines. A pipe’s material rating, pressure class, temperature, fluid compatibility, and local code requirements are separate checks.

The coefficient alone cannot determine a required pipe size. The calculation also needs the actual inside diameter, flow rate, pipe length, elevation changes, fittings, valves, and boundary pressures. Include local losses and check the governing standard. The EPA manual’s listed C-values are starting references for new pipes, not project-specific design inputs. USACE guidance likewise advises selecting a value based on material and age and notes that coefficient tables differ.

Because the exponent on C is not one, choosing a value from the top of a range does not change predicted head loss in a simple linear proportion. Keep the source and assumed condition beside the calculation so another reviewer can reproduce the result. If the material is lined or coated, use a coefficient supported for that specific condition instead of treating the base material as a match.

When a system is existing, use documented design assumptions or measured operating data when available. If the line has unknown history or visible deposits, a new-pipe table may overstate its hydraulic capacity. For a design decision, have a qualified engineer select the method and assumptions, then verify the result against the applicable utility or project criteria.

At a minimum, record whether the line is new or existing, how its inside diameter was determined, which coefficient source was used, and what flow range the calculation represents. A measured pressure or flow test can help validate a model, but a single operating point does not necessarily establish the coefficient for every demand condition. Use an appropriate system model when branches, pumps, elevation, or changing demand affect the result.

For a quick reasonableness check, compare the chosen value with the table’s material and condition, then verify that the units and exponent convention match the software output. A coefficient that appears plausible does not validate the rest of the model.

For related pipe references, see our PVC pipe size chart and nominal pipe size and outside-diameter chart. Our natural-gas pipe sizing guide covers a different fluid and must not be calculated with Hazen-Williams.