Flex vs Rigid Duct Calculator
Compare pressure loss of flex versus rigid duct.
Enter your values
Enter the airflow in CFM.
Enter the duct diameter in in.
Enter the duct length in ft.
Enter the flex friction multiplier in x rigid.
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.
- 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)
At a glance
| What it does | Compare pressure loss of flex versus rigid duct. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Airflow, Duct diameter, Duct length, Flex friction multiplier |
| Main output | Rigid duct loss |
| Formula | ΔP = friction rate x (length / 100) |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Airflow(CFM)
- Enter the airflow in CFM.
- Duct diameter(in)
- Enter the duct diameter in in.
- Duct length(ft)
- Enter the duct length in ft.
- Flex friction multiplier(x rigid)
- Enter the flex friction multiplier in x rigid.
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.
What do I need to enter into the Flex vs Rigid Duct Calculator?
Just 4 values: airflow, duct diameter, duct length, flex friction multiplier. 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 vs Rigid Duct Calculator work out the answer?
Flexible duct commonly has 2-3 times the friction of smooth rigid duct at the same diameter and airflow. It applies the formula ΔP = friction rate x (length / 100) and shows the working so you can check each step by hand.
Is the Flex vs Rigid Duct 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.