Flex Duct CFM Calculator
Realistic airflow through flexible duct.
Reviewed by James Whitfield · How we check our maths
Your values
ⓘ Live results
Results
- Realistic airflow
- 105 CFM
- Rigid duct equivalent
- 137 CFM
- Loss from flex and slack
- 32 CFM
- Duct area
- 0.196 sq ft
Quick answer
Flex duct carries roughly 20–40% less air than rigid pipe of the same diameter, and much less again when compressed.
The formula
CFM = area x velocity x compression factor
Worked example
Worked example
Using the values the calculator loads with.
Inputs
- Duct diameter6 in
- Design velocity700 FPM
- InstallationSlightly slack (typical)
Result
- Realistic airflow105 CFM
- Rigid duct equivalent137 CFM
- Loss from flex and slack32 CFM
- Duct area0.196 sq ft
How to use the Flex Duct CFM Calculator
- 1Enter the flex duct diameter and your design velocity.
- 2Be honest about how well the duct is stretched.
- 3Compare with the CFM that branch needs and upsize if short.
Frequently asked questions
How much CFM through 6 inch flex?
About 90–110 CFM in a real install, versus roughly 140 CFM for rigid 6 inch pipe.
Why does slack matter so much?
Even 4% compression can cut airflow by a fifth because the inner liner ripples.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error