Flex Duct Equivalent Size Calculator
Upsize flex duct to match rigid duct capacity.
Enter your values
ⓘ Live results
Results
- 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)
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.
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.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.