Pipe Friction Loss Calculator (Hazen-Williams)
Enter the pipe material and size, flow in gallons per minute and the pipe length. The calculator returns velocity, friction loss per 100 feet and the total loss in psi and feet of head, using the Hazen-Williams equation.
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The equation
Hazen-Williams: p = 4.52 × Q1.85 ÷ (C1.85 × d4.87) psi per foot, with Q in GPM and d the inside diameter in inches. C is a roughness coefficient (140 copper, 120 steel, 150 PVC). Velocity in feet per second = 0.4085 × Q ÷ d².
Keep velocity below about 8 fps for hot water and 10 fps for cold water to limit noise and erosion. Fittings add equivalent length; allow 20 to 50% for typical runs. The equation applies to water at normal temperatures only.
Worked example
10 GPM through 1 in. type L copper (inside diameter 1.025 in.): velocity is 3.9 fps and friction loss is about 3.0 psi per 100 feet.
Frequently asked questions
Is Hazen-Williams accurate?
It is accurate for water at ordinary temperatures at velocities up to about 10 fps. For other fluids use Darcy-Weisbach.
Which inside diameter is used?
Copper type L, schedule 40 steel and schedule 40 PVC nominal inside diameters.
How do I account for fittings?
Add the equivalent length of each fitting to your pipe length, or add 20 to 50% as an allowance.
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This calculator gives an estimate for planning and is not tax, legal or engineering advice. Confirm code-related results against the code edition adopted in your jurisdiction and with your authority having jurisdiction, and follow the manufacturer's instructions.