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Friction Factor (Moody) Calculator

Find the Darcy friction factor for a pipe from the Reynolds number, internal diameter and absolute roughness — the value you feed into the Darcy-Weisbach head-loss equation. Uses f = 64/Re for laminar flow and the explicit Swamee-Jain approximation of the Colebrook-White equation for turbulent flow.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the Reynolds number Re for the flow (compute it separately from velocity, diameter and kinematic viscosity if you don't have it).
  2. Enter the pipe internal diameter D and the pipe material's absolute roughness ε in the same unit (millimetres here).
  3. Read the Darcy friction factor f, the flow regime (laminar, transitional or turbulent) and the relative roughness ε/D.

How it works

If Re < 2300 the flow is laminar and f = 64/Re, independent of roughness. Otherwise the tool uses the Swamee-Jain equation f = 0.25 / [log₁₀(ε/(3.7·D) + 5.74/Re^0.9)]², an explicit fit to the implicit Colebrook-White equation. The relative roughness ε/D is dimensionless, so ε and D may share any unit. Between Re 2300 and 4000 the value is flagged as transitional.

Worked example

Worked example. For Re = 100000 in a 100 mm pipe with ε = 0.045 mm (commercial steel), ε/D = 0.00045 and the Swamee-Jain equation gives f ≈ 0.020196, a fully turbulent value. A laminar case Re = 1000 gives f = 64/1000 = 0.064 regardless of roughness.

Common mistakes

  • Mixing units for ε and D (e.g. ε in mm and D in m) — they must share a unit because only their ratio ε/D matters.
  • Trusting a single friction factor in the transitional band 2300 < Re < 4000, where the flow flips unpredictably between laminar and turbulent.
  • Confusing the Darcy friction factor returned here with the Fanning factor, which is four times smaller.

Frequently asked questions

What is the Swamee-Jain equation?

It is an explicit algebraic approximation to the implicit Colebrook-White equation for the turbulent Darcy friction factor, accurate to about 1% over the range 5000 ≤ Re ≤ 10⁸ and 10⁻⁶ ≤ ε/D ≤ 10⁻². Because it is explicit it avoids the iteration Colebrook-White needs.

What roughness value should I use?

Use the absolute roughness ε for your pipe material: roughly 0.0015 mm for drawn tubing or PVC, 0.045 mm for commercial steel, 0.15 mm for galvanised iron and 0.3–3 mm for concrete. New pipe is smoother than aged or scaled pipe.

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