Hazen Williams Calculator
Calculate water pipe flow using the Hazen-Williams equation. Solve for velocity, flow rate, pipe diameter, roughness coefficient, or hydraulic slope.
What is the Hazen-Williams Equation?
The Hazen-Williams equation is an empirical formula used exclusively for calculating water flow in pipes at normal temperatures. It relates flow velocity to pipe roughness, hydraulic radius, and the energy grade line slope. Developed for water distribution system design, it is simpler than the Darcy-Weisbach equation but limited to water in turbulent flow. For related hydraulic calculations, try our Manning Equation Calculator and Pipe Flow Calculator.
The equation exists in two common forms: the SI velocity form and the US customary flow form. Both use the same roughness coefficient (C), with higher C values indicating smoother pipes and less friction loss.
How to Use the Hazen-Williams Calculator
Select the equation type (Velocity or Flow) and the variable to solve for. Enter the known values and the calculator instantly computes the result. The SI velocity form outputs in m/s using hydraulic radius, while the US flow form outputs in gpm using pipe diameter.
Hazen-Williams Formulas
v = 0.849 × C × Rh0.63 × S0.54
SI velocity form - v in m/s, Rh in meters
Q = 0.285 × C × D2.63 × S0.54
US flow form - Q in gpm, D in inches
Roughness Coefficient (C) Reference
- New PVC / HDPE: 150
- New copper: 150
- New ductile iron: 130
- New cast iron: 130
- 20-year-old cast iron: 100
- Old/tuberculated iron: 80
- Concrete: 120-140
- Steel: 120
Frequently Asked Questions
What does the Hazen-Williams C coefficient represent?
C is an empirical roughness factor describing pipe interior smoothness. New PVC and copper are ~150, new ductile iron ~130, and 20-year-old cast iron can fall to ~100. Higher C means less friction loss.
When should I use Hazen-Williams versus Darcy-Weisbach?
Hazen-Williams is accurate for water at 4-25 degrees C in turbulent flow inside common municipal piping. Use Darcy-Weisbach for other fluids, high or low temperatures, gas flow, or laminar flow conditions.
How do the SI and US forms differ?
The SI form v = 0.849 x C x Rh^0.63 x S^0.54 outputs velocity in m/s using hydraulic radius in meters. The US form Q = 0.285 x C x D^2.63 x S^0.54 outputs flow in gpm using pipe diameter in inches.
What is hydraulic radius and how does it differ from pipe diameter?
Hydraulic radius is the cross-sectional flow area divided by the wetted perimeter. For a full circular pipe, Rh = D/4, not D/2. For open channels or partially full pipes, compute Rh explicitly from the geometry.
Why is Hazen-Williams only valid for water?
The prefactors absorb water's density and viscosity at room temperature. Using the equation for oil, glycol, or hot water will give wrong results because the implicit viscosity assumption is violated.
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