Fluid Mechanics

Friction Factor Calculator

Q: Does this return Darcy or Fanning friction factor?

This calculator outputs the Darcy (Moody) friction factor only. Fanning friction factor = Darcy / 4. If your source uses Fanning convention, divide the result by 4.

Q: Does this look up pipe roughness?

No. You must supply the absolute pipe roughness directly. For roughness values, consult pipe manufacturer data or engineering references such as Crane TP-410.

Q: What happens in the transitional regime?

For 2300 ≤ Re ≤ 4000, the calculator uses the Swamee-Jain approximation but flags the result as approximate. Friction factor predictions in this range are unreliable. Avoid operating in the transitional zone.

Q: Does this replace a Moody chart?

No. The Swamee-Jain approximation is an explicit formula that approximates the Colebrook-White equation. For critical applications, verify against published Moody charts or iterative Colebrook solutions.

This calculator determines the Darcy friction factor for internal pipe flow. For laminar flow (Re < 2300), it uses f = 64/Re. For turbulent flow (Re > 4000), it uses the Swamee-Jain explicit approximation. The transitional regime (2300 ≤ Re ≤ 4000) is flagged as approximate. You must supply the Reynolds number and absolute roughness — no pipe material, roughness, or viscosity lookup is performed.

TypeInteractive calculator — separate from unit conversions

Calculator

Reynolds number (Re)

Internal diameter (D)

Absolute roughness (ε)

Result

Darcy friction factor (f)0.0194985

Flow regimeTurbulent

Relative roughness (ε/D)0.0003

Formulas

Laminar (Re < 2300)

f = 64 / Re

Turbulent — Swamee-Jain

f = 0.25 / [log₁₀(ε/(3.7D) + 5.74/Re⁰·⁹)]²

Relative roughness

ε/D = ε / D

Diagram

Worked example

Turbulent: Re = 100,000, D = 150 mm, ε = 0.045 mm. Laminar: Re = 1,500.

01Laminar: f = 64 / 1500 = 0.04267. Regime: Laminar.
02Turbulent: ε/D = 0.000045 / 0.15 = 0.0003
03f = 0.25 / [log₁₀(0.000045/(3.7×0.15) + 5.74/100000⁰·⁹)]²
04f ≈ 0.01879. Regime: Turbulent.

Result

Laminar: f = 0.04267. Turbulent: f ≈ 0.01879.

FAQ

Does this return Darcy or Fanning friction factor?

This calculator outputs the Darcy (Moody) friction factor only. Fanning friction factor = Darcy / 4. If your source uses Fanning convention, divide the result by 4.

Does this look up pipe roughness?

No. You must supply the absolute pipe roughness directly. For roughness values, consult pipe manufacturer data or engineering references such as Crane TP-410.

What happens in the transitional regime?

For 2300 ≤ Re ≤ 4000, the calculator uses the Swamee-Jain approximation but flags the result as approximate. Friction factor predictions in this range are unreliable. Avoid operating in the transitional zone.

Does this replace a Moody chart?

No. The Swamee-Jain approximation is an explicit formula that approximates the Colebrook-White equation. For critical applications, verify against published Moody charts or iterative Colebrook solutions.