Darcy-Weisbach Calculator — Pipe Pressure Drop & Head Loss

Formula

How this is calculated

The Darcy-Weisbach equation ΔP = f × (L/D) × (ρv²/2) gives the pressure drop along a straight pipe, where f is the dimensionless Darcy friction factor, L is pipe length, D is internal diameter, ρ is fluid density and v is mean flow velocity. Dividing by (ρg) gives the equivalent head loss hf in metres of fluid. Minor losses (bends, valves, fittings) are not included — this calculator handles major (friction) losses only.

The friction factor depends on the Reynolds number Re = ρvD/μ (μ = dynamic viscosity) and pipe relative roughness ε/D. For laminar flow (Re < 2300), f = 64/Re regardless of roughness. For turbulent flow (Re ≥ 4000), this calculator uses the Swamee-Jain explicit approximation to the Colebrook-White equation: f = 0.25 / [log₁₀(ε/(3.7D) + 5.74/Re^0.9)]². The transitional zone (2300 ≤ Re < 4000) is unpredictable; this calculator linearly interpolates as an approximation. Default fluid properties are for water at 20°C (ρ = 998 kg/m³, μ = 1.002 mPa·s).

Pipe roughness values (ε) are design estimates from published tables (e.g. Moody chart sources): commercial steel ≈ 0.046 mm, galvanised iron ≈ 0.15 mm, cast iron ≈ 0.26 mm, concrete 0.3–3 mm. Actual roughness increases with age, scaling and corrosion; always apply an appropriate safety factor in design.