Pipe Flow Friction Loss Calculator
Calculates pressure drop due to friction in straight pipe runs using the Darcy-Weisbach equation. Use it when sizing pumps, designing piping systems, or troubleshooting flow resistance in water, oil, or gas networks.
Last updated: September 2026
Formula below · 2 sources (asme.org, Wikipedia) · Updated Sep 2026
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About this calculator
The Darcy-Weisbach equation is the industry-standard method for computing frictional pressure drop in pipes: ΔP = f · (L/D) · (ρv²/2), where f is the Darcy friction factor, L is pipe length, D is internal diameter, ρ is fluid density, and v = Q / (πD²/4) is the mean velocity. The friction factor depends on the Reynolds number Re = vD/ν and the relative roughness ε/D. This calculator uses f = 64/Re for laminar flow and the Swamee-Jain equation f = 0.25 / [log₁₀(ε/(3.7D) + 5.74/Re^0.9)]² for turbulent flow (within about 1% of Colebrook-White), with ν = 1.004 × 10⁻⁶ m²/s (water at 20 °C). Roughness values: commercial steel 0.045 mm, galvanized iron 0.15 mm, cast iron 0.26 mm, PVC 0.0015 mm. For other fluids the viscosity differs, so treat the result as a water estimate. Higher roughness, longer pipes, smaller diameters, faster flows, and denser fluids all increase pressure loss; fittings add minor losses not included here.
How to use
Suppose water (ρ = 1000 kg/m³) flows at Q = 0.005 m³/s through a 100 mm commercial steel pipe (ε = 0.045 mm) that is 50 m long. Velocity: v = 0.005 / (π × 0.05²) = 0.637 m/s. Reynolds number: 0.637 × 0.1 / 1.004×10⁻⁶ ≈ 63,400. Friction factor (Swamee-Jain, ε/D = 0.00045): f ≈ 0.0216. ΔP = 0.0216 × (50 / 0.1) × 1000 × 0.637² / 2 ≈ 2,190 Pa, about 2.2 kPa (44 Pa/m). The defaults (0.05 m³/s in 200 mm steel pipe over 100 m) give about 10.4 kPa.
Frequently asked questions
What is the Darcy-Weisbach equation and when should I use it for pipe friction calculations?
The Darcy-Weisbach equation relates frictional pressure loss to pipe geometry, fluid properties, and flow velocity: ΔP = f·(L/D)·(ρv²/2). It is applicable to any fluid—liquid or gas—in laminar or turbulent flow, making it more universally accurate than older empirical formulas like Hazen-Williams. Engineers use it for pump sizing, pipe selection, and system hydraulic analysis. It requires knowing or estimating the Darcy friction factor f, which depends on Reynolds number and relative pipe roughness.
How does pipe roughness affect friction loss in a pipeline system?
Pipe roughness ε represents microscopic surface irregularities on the pipe wall. As roughness increases relative to pipe diameter (the ratio ε/D), the friction factor f rises, directly increasing pressure drop. A smooth plastic pipe has much lower friction losses than an old corroded steel pipe of the same diameter and length. In fully turbulent flow, friction loss becomes nearly independent of flow rate and is controlled almost entirely by roughness, a regime described by the fully rough Moody chart region.
Why does reducing pipe diameter dramatically increase friction pressure losses?
Friction loss is highly sensitive to pipe diameter because two effects combine: reducing D increases flow velocity (v = Q/A, where A ∝ D²), and it also increases the L/D ratio term. Since pressure loss scales with v², halving the pipe diameter increases velocity by a factor of four and pressure loss by roughly sixteen times, all else being equal. This is why engineers upsize pipes in high-flow systems—the energy savings from reduced friction quickly offset the higher material cost.