Darcy Friction Factor Calculator

What Is the Darcy Friction Factor?

The Darcy friction factor $f$ quantifies wall friction in pipe flow. It appears in the Darcy-Weisbach equation for head loss and depends on Reynolds number and pipe roughness.

Laminar and Turbulent Formulas

$$ f = \frac{64}{Re} \quad (Re < 2300) $$

For turbulent flow, the Swamee-Jain equation approximates the implicit Colebrook equation:

$$ f = \frac{0.25}{\left[\log_{10}\left(\frac{\varepsilon/D}{3.7} + \frac{5.74}{Re^{0.9}}\right)\right]^2} $$

$\varepsilon/D$ is relative roughness (absolute roughness divided by pipe diameter). Example: $Re = 10^5$ and $\varepsilon/D = 0.00015$ gives $f \approx 0.018$.

How to Use This Calculator

Enter Reynolds number and relative roughness. The tool selects laminar or turbulent formula automatically and reports the Darcy friction factor.

Frequently Asked Questions

When is flow laminar?

Laminar pipe flow is usually taken for Reynolds number below about 2300. The friction factor then depends only on Re, not roughness.

What is relative roughness?

It is absolute wall roughness epsilon divided by pipe diameter D. Typical steel pipe might be 0.00015 to 0.0005.

Swamee-Jain vs Colebrook?

Swamee-Jain is an explicit approximation to Colebrook, accurate within about 1% for turbulent flow and much faster to evaluate.

Is this the Darcy or Fanning factor?

This calculator returns the Darcy friction factor. Fanning factor equals Darcy f divided by 4.

How do I use f in head loss?

Plug f into the Darcy-Weisbach equation: head loss equals f times L over D times V squared over 2g.

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Friction Factor Calculator

What Is the Darcy Friction Factor?

Laminar and Turbulent Formulas

How to Use This Calculator

Frequently Asked Questions

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