Duct Size Calculator
Round and rectangular duct size for a target CFM.
Enter your values
ⓘ Live results
Results
- Round duct diameter
- 9.6 in
- Next standard size up
- 10 in
- Rectangular option
- 9 x 8 in
- Duct area needed
- 0.5 sq ft
Working
- 1400 CFM / 800 FPM = 0.5 sq ft
Quick answer
Required duct area is CFM divided by design velocity; the diameter follows from that area.
d = 12 x sqrt(4 x CFM / (pi x FPM x 144)) ... area = CFM / FPM
How to use the Duct Size Calculator
- 1Enter the CFM the branch or trunk must carry.
- 2Set the design velocity — lower is quieter, higher saves space.
- 3Round up to the next stocked duct size.
Worked example
Using the values the calculator loads with:
Inputs
- Required airflow400 CFM
- Design velocity800 FPM
- Preferred rect height8 in
Results
- Round duct diameter9.6 in
- Next standard size up10 in
- Rectangular option9 x 8 in
- Duct area needed0.5 sq ft
- 400 CFM / 800 FPM = 0.5 sq ft
Frequently asked questions
What size duct for 400 CFM?
At a quiet 800 FPM you need about 0.5 sq ft, which is roughly a 10 inch round duct.
Can I use a smaller duct with a stronger fan?
You can, but static pressure and noise rise sharply — velocity above 1500 FPM is rarely worth it in a home.
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.