Darcy Friction Factor Calculator — Pipe Flow
Find the Darcy friction factor (f) for internal pipe flow. Enter pipe diameter, flow velocity, fluid kinematic viscosity and wall roughness — the calculator applies the Swamee–Jain explicit approximation (±3 % of Colebrook–White) and labels the flow regime.
m
m/s
cSt
mm
Dimensionless pipe resistance coefficient
- 1
Reynolds number
v × D ÷ ν = 0.5 × 0.1 ÷ 0.000001 = 50,000 - 2
Relative roughness ε/D
0.000046 ÷ 0.1 = 0.00046 - 3
Log argument (Swamee–Jain)
ε/(3.7D) + 5.74/Re⁰·⁹ = 0.000463Sum of roughness and Reynolds-number terms inside the log₁₀. - 4
Friction factor
0.25 ÷ log₁₀(0.000463)² = 0.02249
How does this calculator work?
The Darcy friction factor f = 64/Re for laminar flow (Re < 2 300), and ≈ 0.25 / [log₁₀(ε/(3.7D) + 5.74/Re⁰·⁹)]² for turbulent flow (Swamee–Jain). Enter pipe diameter, velocity, kinematic viscosity in cSt and wall roughness in mm to get f, Re and the flow regime.
Formula
How this is calculated
The Darcy–Weisbach friction factor f appears in the head-loss equation ΔP = f·(L/D)·(ρv²/2) and tells you how much energy the pipe wall resists per unit length relative to the kinetic energy of the flow. Its value depends entirely on the Reynolds number Re = vD/ν and the relative roughness ε/D.
For laminar flow (Re < 2 300) the velocity profile is parabolic, viscosity dominates and the exact solution is f = 64/Re. For fully turbulent flow (Re > 4 000) the friction factor follows the Colebrook–White implicit equation; this calculator uses the Swamee–Jain explicit approximation f = 0.25 / [log₁₀(ε/(3.7D) + 5.74/Re⁰·⁹)]², which is accurate to within ±3 % for 5 × 10³ ≤ Re ≤ 10⁸ and 10⁻⁶ ≤ ε/D ≤ 10⁻². The transitional regime (2 300–4 000) is interpolated and physically unreliable — treat those results as estimates only.
Kinematic viscosity is entered in centistokes (cSt), where 1 cSt = 10⁻⁶ m²/s; water at 20 °C is about 1.0 cSt, air at 20 °C about 15 cSt, and light hydraulic oil about 46 cSt. Pipe roughness ε is in millimetres: 0.046 mm for commercial steel, 0.26 mm for cast iron, and 0.0015 mm for drawn tubing or smooth glass.
Frequently asked questions
The Darcy (Moody) friction factor f = 4 × Fanning friction factor. The Darcy form is used in the Darcy–Weisbach head-loss equation and is the more common one in engineering. Always confirm which convention a reference uses before substituting values.
Swamee–Jain is an explicit approximation that avoids the iterative solution required by Colebrook–White. The error is within ±3 % over the valid range (5 000 ≤ Re ≤ 10⁸, 10⁻⁶ ≤ ε/D ≤ 10⁻²), which is acceptable for most engineering calculations.
Typical absolute roughness values: commercial/welded steel 0.046 mm, cast iron 0.26 mm, galvanised steel 0.15 mm, PVC/drawn copper 0.0015 mm, concrete 0.3–3 mm. Use manufacturer data where available — age and fouling increase roughness significantly.
Also known as
TG we-Calculate Editorial Team. (2026). Darcy Friction Factor Calculator — Pipe Flow [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/friction-factor-calculator
TG we-Calculate Editorial Team. "Darcy Friction Factor Calculator — Pipe Flow." TG we-Calculate. 2026. https://we-calculate.com/calculator/friction-factor-calculator.
TG we-Calculate Editorial Team, "Darcy Friction Factor Calculator — Pipe Flow," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/friction-factor-calculator
@misc{wecalculate_friction_factor_calculator, title = {Darcy Friction Factor Calculator — Pipe Flow}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/friction-factor-calculator}}, year = {2026}, note = {TG we-Calculate} }
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