Flex Duct Equivalent Size Calculator
Upsize flex duct to match rigid duct capacity.
Enter your values
Enter the rigid duct diameter in in.
Enter the flex compression in %.
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.
- Equivalent flex diameter
- 9 in
- Recommended nominal size
- 9 in
- Extra friction if kept same size
- ~50% higher pressure drop when fully extended, more if compressed
Quick answer
Because flexible duct has higher friction, it typically needs to be sized one size larger than smooth rigid duct for the same airflow and pressure drop.
Flex diameter ≈ rigid diameter x (roughness correction factor)
At a glance
| What it does | Upsize flex duct to match rigid duct capacity. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Rigid duct diameter, Flex compression |
| Main output | Equivalent flex diameter |
| Formula | Flex diameter ≈ rigid diameter x (roughness correction factor) |
| Cost | Free — no sign-up, no download |
How to use the Flex Duct Equivalent Size Calculator
- 1Enter the rigid duct size that meets your airflow target.
- 2Add expected compression, since flex duct is rarely pulled fully taut.
- 3Round up to the nearest stock flex duct size.
Inputs explained
- Rigid duct diameter(in)
- Enter the rigid duct diameter in in.
- Flex compression(%)
- Enter the flex compression in %.
Worked example
Using the values the calculator loads with:
Inputs
- Rigid duct diameter8 in
- Flex compression4 %
Results
- Equivalent flex diameter9 in
- Recommended nominal size9 in
- Extra friction if kept same size~50% higher pressure drop when fully extended, more if compressed
Frequently asked questions
Why is flex duct worse than rigid?
Its corrugated wall roughness and sag between supports both add friction compared to smooth metal.
Does stretching flex help?
Yes, fully extending flex duct significantly cuts its friction versus a compressed or sagging run.
What do I need to enter into the Flex Duct Equivalent Size Calculator?
Just 2 values: rigid duct diameter, flex compression. 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 Equivalent Size Calculator work out the answer?
Because flexible duct has higher friction, it typically needs to be sized one size larger than smooth rigid duct for the same airflow and pressure drop. It applies the formula Flex diameter ≈ rigid diameter x (roughness correction factor) and shows the working so you can check each step by hand.
Is the Flex Duct Equivalent Size 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.