Friction Factor Calculator
Calculate Darcy friction factor (f) from Reynolds number and relative roughness.
How to Use This Calculator
Enter Reynolds number
Use Re = ρvD/μ with consistent SI units.
Enter ε and D
Provide absolute roughness ε (m) and diameter D (m) to compute ε/D.
Compute f
Laminar: f = 64/Re. Turbulent: Swamee–Jain gives an explicit approximation.
Formula
Laminar: f = 64/Re
Turbulent (Swamee–Jain): f = 0.25 / [log10((ε/D)/3.7 + 5.74/Re^0.9)]^2
Where: Re = Reynolds number, ε = absolute roughness (m), D = pipe diameter (m).
Example: Re = 100000, ε = 0.00015 m, D = 0.1 m → ε/D = 0.0015 → f ≈ 0.020.
About Friction Factor Calculator
The Darcy friction factor is used in the Darcy–Weisbach equation to estimate head loss in pipe flow. It depends primarily on Reynolds number and relative roughness.
Notes
- Transitional regime (Re ≈ 2300–4000) is not well captured by explicit formulas.
- For high accuracy in turbulent regime, a Colebrook iteration can be used.
Frequently Asked Questions
Is this the Darcy or Fanning factor?
This calculator returns the Darcy friction factor (4× the Fanning factor).
What if I only know material?
Look up typical ε for the pipe material and size, then compute ε/D.
Can I use imperial units?
Yes, so long as ε and D use the same length units; Re is dimensionless.