The Darcy (and Fanning) friction factor for pipe flow — from Reynolds number and roughness, or straight from your pipe and fluid. Solved with the full Colebrook equation, not just a chart read.
The Number Behind Every Head-Loss Calculation
The friction factor is the dimensionless number that captures how hard a pipe resists flow. Feed it Darcy-Weisbach and you get pressure drop; it is the heart of nearly every pipe-sizing calculation. It depends on just two things: the Reynolds number (is the flow laminar or turbulent?) and the relative roughness of the pipe wall.
Darcy or Fanning? Know Which You Have
This is the classic trap. The Darcy (Darcy-Weisbach, or Moody) friction factor and the Fanning friction factor describe the same physics but differ by a factor of four:
Use the Darcy factor with the Darcy-Weisbach equation and the Fanning factor with the Fanning equation — mixing them gives an answer wrong by 4x or 16x. The calculator shows both so you can match your formula.
Laminar vs Turbulent
Below a Reynolds number of about 2300 the flow is laminar and the friction factor is exactly f = 64/Re — roughness does not matter. Above it the flow is turbulent and the factor follows the implicit Colebrook equation, which this tool solves by iteration rather than reading off a Moody chart. Between 2300 and 4000 lies the transition zone, where any value is approximate.
Frequently Asked Questions
What is relative roughness?
The pipe wall roughness divided by the inside diameter, a dimensionless ratio. Smooth drawn tubing is near 0.000001; old cast iron can exceed 0.01. Use the pipe-and-fluid mode to compute it from an absolute roughness.
Why solve Colebrook iteratively?
Colebrook is implicit — the friction factor appears on both sides — so it cannot be solved in one step. The calculator iterates to convergence, which is more accurate than explicit approximations like Swamee-Jain.
Does roughness matter in laminar flow?
No. In laminar flow the friction factor is purely 64/Re; wall roughness only affects turbulent flow.
Related calculators
- Pressure Drop Calculator — put this factor to work in Darcy-Weisbach.
- Reynolds Number Calculator — the regime input this factor needs.
- Pipe Flow Calculator — flow, velocity and sizing.
- Pump Horsepower Calculator — size the pump for the resulting loss.