Pipe Friction Factor Calculator
The Darcy friction factor from pipe diameter, wall roughness and Reynolds number, using the Swamee-Jain equation.
Results
What this tool does
The friction factor is the one number the Darcy-Weisbach equation cannot give you: it depends on how rough the pipe wall is compared with the bore, and on how turbulent the flow is. Read it off a Moody chart, or let this explicit fit to the Colebrook curve do it. Below Re 2300 the flow is laminar and none of this applies — the laminar value 64/Re shown alongside is the one to use.
Formula
f = 0.25 / [log10(k/(3.7 D) + 5.74/Re^0.9)]^2 (Swamee-Jain)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
dd | Pipe diameter | mm |
ee | Wall roughness | mm |
re | Reynolds number | — |
FF | Friction factor | — |
RR | Relative roughness k/D | — |
FL | Laminar value 64/Re | — |
FS | Value for a smooth pipe | — |
Worked example
- Pipe diameter100 mm
- Wall roughness0.045 mm
- Reynolds number100,000
- Friction factor0.02020
- Relative roughness k/D0.000450
- Laminar value 64/Re0.00064
- Value for a smooth pipe0.01786
Limitations
- Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
Frequently asked questions
Why not solve Colebrook directly?
Colebrook is implicit: the friction factor appears on both sides and has to be found by iteration. The Swamee-Jain equation is an explicit fit to it, and stays within about 1% of the Colebrook answer over the range 5000 < Re < 10^8 and 10^-6 < k/D < 10^-2.