Pipe Friction Loss Calculator (Darcy-Weisbach)

The Darcy-Weisbach equation: ΔP = f × (L/D) × (ρ × v²/2), where f is the Darcy friction factor, L is pipe length, D is internal diameter, ρ is fluid density, and v is mean flow velocity. For the friction factor, laminar flow (Re < 2300) uses f = 64/Re exactly. Turbulent flow uses the Swamee-Jain (1976) explicit approximation: f = 0.25 / [log₁₀(ε/(3.7D) + 5.74/Re⁰·⁹)]². This avoids the iteration required by Colebrook-White.

Swamee-Jain (1976) is an explicit approximation of the implicit Colebrook-White equation. It is accurate to within ±3% for Re between 5×10³ and 10⁸ and relative roughness ε/D between 10⁻⁶ and 0.05 — which covers virtually all industrial piping. It is widely used in process engineering and is accepted by CRANE Technical Paper 410 as a reliable explicit alternative to Colebrook-White.

They express the same energy loss in different units. Head loss (hf, in metres or feet of fluid) is independent of fluid density: hf = ΔP / (ρ × g). Pressure drop (ΔP, in Pa, bar, or psi) depends on fluid density. For water at 20°C, 1 m head = 9,789 Pa ≈ 1.42 psi. For oil at SG 0.85, the same 1 m head = 8,321 Pa ≈ 1.21 psi. Use head loss when sizing pumps; use pressure drop when sizing control valves.

Absolute roughness (ε) depends on pipe material and condition: new commercial steel ≈ 0.046 mm; used/corroded steel ≈ 0.26 mm; galvanized steel ≈ 0.15 mm; stainless steel ≈ 0.046 mm; PVC / HDPE ≈ 0.0015 mm; cast iron ≈ 0.26 mm. These are reference values from CRANE TP-410 and Moody (1944). For aged process pipe, use the corroded steel value unless you have cleaning or pigging records.

No — this calculator covers major (friction) losses in straight pipe only. For minor losses, use the equivalent-length method: add each fitting's equivalent length (Le) to the pipe length L before calculating, or use the K method: ΔP_minor = K × ρv²/2. A 2″ 90° standard elbow has Le ≈ 1.5 m; a 2″ fully-open ball valve Le ≈ 0.15 m. For systems with many fittings, minor losses can exceed 20% of total ΔP.

Re < 2300: laminar (f = 64/Re applies exactly). Re 2300–4000: transition zone — flow alternates between laminar and turbulent, and the Swamee-Jain friction factor is unreliable. Re > 4000: turbulent, Swamee-Jain applies. In practice, most industrial pipe flow at normal velocities (>0.3 m/s) is turbulent unless the fluid is highly viscous.

1 m³/h = 16.667 L/min = 4.403 gpm. 1 gpm = 0.2271 m³/h = 3.785 L/min. For velocity: v = Q / A where A = π D²/4. Example: Q = 10 m³/h in a 2" Sch 40 pipe (ID = 52.5 mm) → v = (10/3600) / (π × 0.0525² / 4) ≈ 1.28 m/s.

Between Re = 2300 and Re = 4000, flow is unstable — it can be laminar or turbulent depending on pipe entry conditions and disturbances. The friction factor is unpredictable. CRANE TP-410 recommends conservatively treating transition-zone flow as turbulent when designing for pressure drop. The calculator shows this warning so you can recognise that the result has higher uncertainty in this range.