Spiral Duct Size Calculator
Round spiral duct size from CFM.
Enter your values
Enter the airflow in CFM.
Enter the design velocity in FPM.
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.
- Spiral duct diameter
- 12.8 in
- Nominal stock size
- 13 in
- Cross-section area
- 0.889 sq ft
Quick answer
Spiral duct is sized the same as round duct by diameter, but its smooth seam gives friction close to smooth rigid duct.
Diameter = sqrt(4 x CFM / (velocity x pi)) x 12
At a glance
| What it does | Round spiral duct size from CFM. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Airflow, Design velocity |
| Main output | Spiral duct diameter |
| Formula | Diameter = sqrt(4 x CFM / (velocity x pi)) x 12 |
| Cost | Free — no sign-up, no download |
How to use the Spiral Duct Size Calculator
- 1Enter the design CFM for the spiral duct run.
- 2Use a velocity appropriate to the application — 900-1500 FPM is common for exposed commercial spiral duct.
- 3Round to the nearest available spiral duct stock diameter.
Inputs explained
- Airflow(CFM)
- Enter the airflow in CFM.
- Design velocity(FPM)
- Enter the design velocity in FPM.
Worked example
Using the values the calculator loads with:
Inputs
- Airflow800 CFM
- Design velocity900 FPM
Results
- Spiral duct diameter12.8 in
- Nominal stock size13 in
- Cross-section area0.889 sq ft
Frequently asked questions
Why use spiral duct instead of rectangular?
It's cheaper per CFM to fabricate, has lower friction than most rectangular duct, and is often left exposed as a design feature.
Does spiral duct need insulation?
Yes if it runs through unconditioned space or needs acoustic or thermal control, using an external wrap or liner.
What do I need to enter into the Spiral Duct Size Calculator?
Just 2 values: airflow, design velocity. 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 Spiral Duct Size Calculator work out the answer?
Spiral duct is sized the same as round duct by diameter, but its smooth seam gives friction close to smooth rigid duct. It applies the formula Diameter = sqrt(4 x CFM / (velocity x pi)) x 12 and shows the working so you can check each step by hand.
Is the Spiral Duct 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.