Friction Factor Calculator

Estimate the Darcy friction factor for a pipe from Reynolds number and relative roughness.

About Friction Factor Calculator

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.

Formula

Friction Factor

Turbulent: f = 0.25 ÷ [log₁₀(ε/3.7D + 5.74/Re⁰·⁹)]² (Swamee-Jain); laminar: f = 64/Re

ε/D: Relative roughness, absolute roughness divided by diameter
Swamee-Jain: Explicit approximation to the implicit Colebrook equation

Worked example

With the default values below, Friction Factor Calculator returns Darcy Friction Factor of 0.020196. Change any input above and every figure here is recalculated in your browser.

Inputs used

Reynolds numberminimum 1: 100,000
Pipe diameterminimum 0 m: 0.1 m
Absolute pipe roughnessminimum 0 m: 0.000045 m

Results

Darcy Friction Factor: 0.020196
Relative Roughness: 0.00045000
Flow Regime: Turbulent
Laminar Friction Factor for Comparison: 0.000640
Fully Rough Friction Factor: 0.016311

Step by step

1Values used
2Friction Factor
3Darcy Friction Factor= 0.020196
4Relative Roughness= 0.00045000
5Flow Regime= Turbulent
6Laminar Friction Factor for Comparison= 0.000640
7Fully Rough Friction Factor= 0.016311

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 8 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Friction Factor calculated?

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.

Why does Friction Factor matter?

At high Reynolds numbers friction factor becomes independent of Reynolds number and depends only on roughness, which is the fully rough regime.

What values do I need to enter?

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.