Flex Duct CFM Calculator
Realistic airflow through flexible duct.
Enter your values
Enter the duct diameter in in.
Enter the design velocity in FPM.
Choose one of: Fully stretched, Slightly slack (typical), Visibly compressed.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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.
CFM = area x velocity x compression factor
At a glance
| What it does | Realistic airflow through flexible duct. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Duct diameter, Design velocity, Installation |
| Main output | Realistic airflow |
| Formula | CFM = area x velocity x compression factor |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Duct diameter(in)
- Enter the duct diameter in in.
- Design velocity(FPM)
- Enter the design velocity in FPM.
- Installation
- Choose one of: Fully stretched, Slightly slack (typical), Visibly compressed.
Worked example
Using the values the calculator loads with:
Inputs
- Duct diameter6 in
- Design velocity700 FPM
- InstallationSlightly slack (typical)
Results
- Realistic airflow105 CFM
- Rigid duct equivalent137 CFM
- Loss from flex and slack32 CFM
- Duct area0.196 sq ft
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.
What do I need to enter into the Flex Duct CFM Calculator?
Just 3 values: duct diameter, design velocity, installation. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Flex Duct CFM Calculator work out the answer?
Flex duct carries roughly 20–40% less air than rigid pipe of the same diameter, and much less again when compressed. It applies the formula CFM = area x velocity x compression factor and shows the working so you can check each step by hand.
Is the Flex Duct CFM Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.