Spiral Duct Size Calculator
Round spiral duct size from CFM.
Enter your values
ⓘ Live results
Results
- 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
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.
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.
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.