Water
Darcy-Weisbach Head Loss Calculator
Calculate pipe friction head loss, pressure loss and Darcy friction factor from flow rate, pipe size and roughness.
Use the actual internal diameter.
Representative values only. Actual roughness changes with pipe material, condition and age.
Friction Head Loss
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Darcy-Weisbach friction loss across the entered straight pipe length.
Head loss
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Head loss
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Pressure loss
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Pressure loss
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Pressure loss
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Head loss per 100 m
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Flow velocity
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Reynolds number
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Darcy friction factor
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Relative roughness
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Method:
Darcy-Weisbach head loss:
hf =
f × (L ÷ D) × V² ÷ (2g)
where f is the Darcy friction factor, L is pipe length, D is internal diameter and V is average flow velocity.
Reynolds number:
Re = V × D ÷ ν
For laminar flow, the calculator uses:
f = 64 ÷ Re
For turbulent flow, the Swamee-Jain approximation is used:
f = 0.25 ÷
[log₁₀(ε ÷ 3.7D + 5.74 ÷ Re⁰·⁹)]²
where ε is absolute pipe roughness.
For the transitional region, the calculator blends the laminar and turbulent friction-factor estimates.
The result includes straight-pipe friction only. Losses through valves, elbows, tees, entrances and other fittings must be added separately when significant.