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Darcy-Weisbach Pipe Friction Loss Calculator

Find the head loss and pressure drop a fluid loses to friction flowing through a straight pipe, h_f = f × (L/D) × (v²/2g), from the Darcy friction factor, pipe geometry, and flow velocity.

Head loss
4.0789 m
Head loss (ft)
13.3821 ft
Pressure drop
40 kPa
Pressure drop (psi)
5.802 psi

How it works

The Darcy-Weisbach equation is the standard way to find how much pressure a fluid loses to friction flowing through a straight pipe: h_f = f × (L/D) × (v²/2g), where f is the dimensionless Darcy friction factor, L and D are the pipe's length and diameter, v is the mean flow velocity, and g is standard gravity (9.80665 m/s²). The friction factor itself normally comes from a Moody chart or the Colebrook equation, which both depend on the pipe's relative roughness and Reynolds number — you supply f directly here (this site's existing Reynolds Number calculator can help estimate the flow regime), rather than this calculator guessing a roughness value for your specific pipe.

Multiplying the head loss by the fluid's density and g converts it to a pressure drop: ΔP = ρ × g × h_f, which simplifies algebraically to the equivalent ΔP = f × (L/D) × (ρv²/2) form many engineering references state directly. Length, diameter, and velocity are converted to canonical meters/meters-per-second internally before the formula runs regardless of the unit toggle; fluid density is left in the universally-used kg/m³ either way, the same convention this site's Hydrostatic Pressure and Venturi Flow Rate calculators already use.

FAQ

Where do I get the Darcy friction factor?

For laminar flow (Reynolds number below ~2300) it has the closed form f = 64/Re. For turbulent flow it depends on both the Reynolds number and the pipe's relative roughness (roughness height ÷ diameter), typically read from a Moody chart or solved from the implicit Colebrook equation — values for smooth drawn pipe commonly run around 0.02-0.03 for everyday turbulent water flow, which is why that's this calculator's default.

Why does halving the pipe diameter matter so much?

Head loss scales as 1/D⁵ once you account for the velocity needed to carry the same flow rate through a narrower pipe (velocity itself scales as 1/D² for constant volumetric flow), so even a modest reduction in diameter produces a sharp rise in friction loss — the reason undersized pipework causes disproportionately large pressure problems in real plumbing and process-piping systems.

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