Friction Loss Calculator — Darcy–Weisbach Head Loss
Calculate the friction head loss (m) and pressure drop (Pa, kPa) in a straight pipe using the Darcy–Weisbach equation. Enter the Darcy friction factor, pipe length and diameter, flow velocity and fluid density.
m
m
m/s
kg/m³
Darcy–Weisbach pipe friction head loss
- 1
L/D ratio
100 ÷ 0.1 = 1,000 - 2
Velocity head v²/(2g)
1.5² ÷ (2 × 9.81) = 2.25 ÷ 19.62 = 0.1147 - 3
Head loss hf
f × (L/D) × v²/(2g) = 0.023 × 1,000 × 0.1147 = 2.638
How does this calculator work?
Pipe friction head loss: hf = f × (L/D) × v²/(2g), where f is the Darcy friction factor, L pipe length, D diameter, v velocity, g = 9.81 m/s². Pressure drop ΔP = ρ g hf. Enter the five inputs to get head loss in metres and pressure drop in kPa.
Formula
How this is calculated
The Darcy–Weisbach equation is the standard model for friction losses in straight pipe sections: hf = f × (L/D) × v²/(2g), where hf is the head loss in metres, f is the dimensionless Darcy friction factor, L and D are the pipe length and inner diameter (both in metres), v is the mean flow velocity in m/s, and g = 9.81 m/s². The term v²/(2g) is the velocity head — the kinetic energy of the flow per unit weight — and the ratio L/D scales it up by how many diameters long the pipe is. The friction factor f amplifies the result depending on turbulence and wall roughness; use the friction factor calculator to find f from pipe material and flow conditions.
Pressure drop in Pascals follows directly from head loss: ΔP = ρ × g × hf, where ρ is the fluid density in kg/m³ (water ≈ 1 000, air ≈ 1.2, sea water ≈ 1 025). The result is the frictional component of pressure loss only — it excludes minor losses from bends, valves, fittings and sudden expansions, which must be added separately using loss-coefficient methods.
The plot shows head loss growing linearly with pipe length at the current conditions, which confirms the Darcy–Weisbach formula is linear in L. Doubling the pipe length doubles the head loss; doubling the velocity quadruples it (v² dependence). These scaling rules are useful for quick sanity checks.
Frequently asked questions
Head loss (m) is pressure drop expressed as a height of fluid column: ΔP = ρ × g × hf. Head loss is independent of fluid density and is often more convenient for hydraulic calculations across systems with different fluids.
No — it covers straight-pipe friction losses only. Add minor losses separately using K × v²/(2g) for each fitting, where K is the loss coefficient from tables (e.g., K ≈ 0.75 for a standard elbow, K ≈ 10 for a globe valve).
Head loss scales as 1/D × v² — reducing diameter increases loss both by the 1/D factor and by raising velocity (for the same flow rate Q = v × π D²/4, so v ∝ 1/D²). Overall hf ∝ 1/D⁵ for constant Q, so even small diameter reductions cause large increases in loss.
Also known as
TG we-Calculate Editorial Team. (2026). Friction Loss Calculator — Darcy–Weisbach Head Loss [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/friction-loss-calculator
TG we-Calculate Editorial Team. "Friction Loss Calculator — Darcy–Weisbach Head Loss." TG we-Calculate. 2026. https://we-calculate.com/calculator/friction-loss-calculator.
TG we-Calculate Editorial Team, "Friction Loss Calculator — Darcy–Weisbach Head Loss," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/friction-loss-calculator
@misc{wecalculate_friction_loss_calculator, title = {Friction Loss Calculator — Darcy–Weisbach Head Loss}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/friction-loss-calculator}}, year = {2026}, note = {TG we-Calculate} }
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