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Guide

Darcy-Weisbach vs Hazen-Williams: Which to Use

When calculating friction losses in pipe networks, engineers typically choose between two well-established methods: Darcy-Weisbach and Hazen-Williams. Both predict head loss due to friction, but they differ significantly in their assumptions, accuracy, and range of applicability.

Darcy-Weisbach Method

The Darcy-Weisbach equation is considered the most theoretically rigorous approach to calculating friction losses. It uses the Colebrook-White equation to determine the friction factor based on Reynolds number and relative pipe roughness.

The head loss formula is:

hf=f⋅LD⋅V22gh_f = f \cdot \frac{L}{D} \cdot \frac{V^2}{2g}
  • hfh_f — head loss due to friction
  • ff — Darcy friction factor
  • LL — pipe length
  • DD — internal diameter
  • VV — flow velocity
  • gg — gravitational acceleration

Strengths

  • Works for any Newtonian fluid (water, oil, gas, refrigerants)
  • Valid across all flow regimes (laminar, transitional, turbulent)
  • Accounts for fluid viscosity and pipe roughness independently
  • Considered the most accurate method for engineering calculations

Limitations

  • Requires iterative solution of the Colebrook-White equation
  • Needs fluid viscosity as an input (temperature-dependent)

Hazen-Williams Method

The Hazen-Williams formula is an empirical equation widely used in water distribution system design. It uses a single C-factor (roughness coefficient) to characterise pipe friction.

hf=10.67⋅L⋅Q1.852C1.852⋅D4.8704h_f = \frac{10.67 \cdot L \cdot Q^{1.852}}{C^{1.852} \cdot D^{4.8704}}
  • hfh_f — head loss due to friction
  • CC — Hazen-Williams coefficient (typically 100-150)
  • QQ — flow rate
  • DD — internal diameter
  • LL — pipe length

Strengths

  • Simple, non-iterative calculation
  • C-factors are well-documented for common pipe materials
  • Industry standard for municipal water system design
  • Used by EPANET and many water distribution modelling tools

Limitations

  • Only valid for water near room temperature (roughly 5-25 C)
  • Only valid for turbulent flow in pipes larger than about 50mm
  • Cannot be used with gases or non-water liquids
  • Less accurate than Darcy-Weisbach at extreme velocities

Which Should You Choose?

Use Darcy-Weisbach when working with gases, non-water liquids, high-temperature water, small-diameter pipes, or when maximum accuracy is required. It is the recommended default for general-purpose pipe network simulation.

Use Hazen-Williams when designing municipal water distribution networks, importing EPANET models, or when project specifications require it. Many water utility standards and local codes reference H-W coefficients directly.

In SimuPipe, you can switch between both models using the friction model selector in the simulation toolbar. Pipe roughness values and H-W C-factors are stored independently, so switching models does not lose any data. You can also try both methods side-by-side using our free Pipe Friction Loss Calculator.