Duct Size Calculator
Duct diameter for a target airflow.
Enter your values
Enter the airflow in CFM.
Enter the target velocity in ft/min.
Enter the rectangular duct 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
- 11.1 in
- Duct area
- 96 in²
- Rectangular width
- 12 in
- Next standard size
- 12 in
Quick answer
Duct size follows from airflow divided by the target air velocity.
Area = CFM ÷ velocity
At a glance
| What it does | Duct diameter for a target airflow. |
|---|---|
| Category | Engineering |
| Inputs needed | Airflow, Target velocity, Rectangular duct height |
| Main output | Round duct diameter |
| Formula | Area = CFM ÷ velocity |
| Cost | Free — no sign-up, no download |
How to use the Duct Size Calculator
- 1Enter the airflow the branch must carry.
- 2Choose a target velocity for noise control.
- 3Enter duct height for a rectangular equivalent.
Inputs explained
- Airflow(CFM)
- Enter the airflow in CFM.
- Target velocity(ft/min)
- Enter the target velocity in ft/min.
- Rectangular duct height(in)
- Enter the rectangular duct height in in.
Worked example
Using the values the calculator loads with:
Inputs
- Airflow600 CFM
- Target velocity900 ft/min
- Rectangular duct height8 in
Results
- Round duct diameter11.1 in
- Duct area96 in²
- Rectangular width12 in
- Next standard size12 in
Frequently asked questions
What velocity is quiet?
700–900 ft/min for residential supply ducts.
Why not use smaller ducts?
Velocity noise and static pressure rise sharply.
What do I need to enter into the Duct Size Calculator?
Just 3 values: airflow, target velocity, rectangular duct 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?
Duct size follows from airflow divided by the target air velocity. It applies the formula Area = CFM ÷ velocity 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.