Flex vs Rigid Duct Calculator
Compare pressure loss of flex versus rigid duct.
Enter your values
ⓘ Live results
Results
- Rigid duct loss
- 0.085 in wc
- Flex duct loss
- 0.212 in wc
- Velocity
- 561 FPM
- Extra loss with flex
- 0.127 in wc
Quick answer
Flexible duct commonly has 2-3 times the friction of smooth rigid duct at the same diameter and airflow.
ΔP = friction rate x (length / 100)
How to use the Flex vs Rigid Duct Calculator
- 1Enter the airflow, diameter and run length.
- 2Use a friction multiplier of 2-3x for typical corrugated flex versus smooth rigid.
- 3Compare the two pressure losses to see if the flex run needs a larger diameter.
Worked example
Using the values the calculator loads with:
Inputs
- Airflow150 CFM
- Duct diameter7 in
- Duct length25 ft
- Flex friction multiplier2.5 x rigid
Results
- Rigid duct loss0.085 in wc
- Flex duct loss0.212 in wc
- Velocity561 FPM
- Extra loss with flex0.127 in wc
Frequently asked questions
Is this exact?
It is an engineering approximation; manufacturer friction charts give more precise figures.
When should I avoid flex entirely?
On long trunk runs or where static pressure budget is already tight, rigid duct is the safer choice.
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.