Darcy Friction Factor Calculator (Colebrook)
Find the Darcy friction factor for pipe flow. The Colebrook White equation is solved by iteration and compared with the explicit Swamee Jain formula; laminar flow uses 64 ÷ Re.
- ε/D = 0.045 mm ÷ 100 mm = 0.00045
- 1/√f = −2 log₁₀(ε/3.7D + 2.51/(Re√f)), solved by iteration: f = 0.02012
Friction Factor Equations
Swamee Jain: f = 0.25 ÷ [log₁₀(ε/(3.7D) + 5.74/Re0.9)]²
Laminar: f = 64 ÷ Re
f is the Darcy friction factor, ε the absolute pipe roughness, D the inside diameter and Re the Reynolds number. The Fanning factor used in some textbooks is f ÷ 4. Roughness values are typical figures for new pipe.
Worked example
At Re = 100,000 in a 100 mm commercial steel pipe (ε = 0.045 mm, ε/D = 0.00045), Colebrook gives f = 0.0201; Swamee Jain gives 0.0202, within 0.4 %.
Frequently Asked Questions
What is the Darcy friction factor?
A dimensionless factor in the Darcy Weisbach equation that represents wall friction. Pressure drop = f × (L/D) × ρv²/2.
Darcy or Fanning friction factor?
The Darcy factor is four times the Fanning factor. Check which one a chart or formula uses before mixing them.
Why does Colebrook need iteration?
The friction factor appears on both sides of the equation. Explicit formulas such as Swamee Jain approximate it within about 1 % for most turbulent flows.
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