Duct Size Calculator
Round and rectangular duct size for a target CFM.
Enter your values
Enter the required airflow in CFM.
Enter the design velocity in FPM.
Enter the preferred rect height in in.
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.
- 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
At a glance
| What it does | Round and rectangular duct size for a target CFM. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Required airflow, Design velocity, Preferred rect height |
| Main output | Round duct diameter |
| Formula | d = 12 x sqrt(4 x CFM / (pi x FPM x 144)) ... area = CFM / FPM |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Required airflow(CFM)
- Enter the required airflow in CFM.
- Design velocity(FPM)
- Enter the design velocity in FPM.
- Preferred rect height(in)
- Enter the preferred rect height in in.
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.
What do I need to enter into the Duct Size Calculator?
Just 3 values: required airflow, design velocity, preferred rect height. 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 Duct Size Calculator work out the answer?
Required duct area is CFM divided by design velocity; the diameter follows from that area. It applies the formula d = 12 x sqrt(4 x CFM / (pi x FPM x 144)) ... area = CFM / FPM and shows the working so you can check each step by hand.
Is the 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.