Estimate the Darcy friction factor for a pipe from Reynolds number and relative roughness.
The Colebrook equation is implicit in the friction factor, so the explicit Swamee-Jain approximation is used instead — it agrees to within about one per cent across the practical range. In laminar flow roughness is irrelevant. At high Reynolds numbers friction factor becomes independent of Reynolds number and depends only on roughness, which is the fully rough regime.
Friction Factor
Turbulent: f = 0.25 ÷ [log₁₀(ε/3.7D + 5.74/Re⁰·⁹)]² (Swamee-Jain); laminar: f = 64/Re
Turbulent: f = 0.25 ÷ [log₁₀(ε/3.7D + 5.74/Re⁰·⁹)]² (Swamee-Jain); laminar: f = 64/Re The Colebrook equation is implicit in the friction factor, so the explicit Swamee-Jain approximation is used instead — it agrees to within about one per cent across the practical range. In laminar flow roughness is irrelevant.
At high Reynolds numbers friction factor becomes independent of Reynolds number and depends only on roughness, which is the fully rough regime.
This calculator takes 3 inputs: Reynolds number, Pipe diameter, Absolute pipe roughness. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.