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Darcy Friction Factor Calculator — Reynolds, Roughness & Colebrook
Calculate Darcy friction factor from Reynolds number and relative roughness using laminar 64/Re or the explicit Swamee-Jain turbulent approximation.
Last updated: September 17, 2026
Results
Enter positive Reynolds number/diameter and non-negative roughness
Formula, example and assumptions
Laminar f=64/Re; turbulent f≈0.25/[log10(ε/(3.7D)+5.74/Re^0.9)]².
At Re=100,000 and a small relative roughness, the Darcy factor is typically a few hundredths.
- The transition region is not represented by one universally reliable simple correlation.
- This calculator reports the Darcy-Weisbach factor, not the Fanning factor.
FAQs
Darcy or Fanning friction factor?
This owner uses Darcy friction factor. The Fanning factor is one quarter of the Darcy value.
Why enter roughness and diameter?
Their ratio is relative roughness, which affects turbulent pipe friction.