Zone Damper Size Calculator
Zone damper size for a zoned duct system.
Enter your values
Enter the zone cfm 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.
- Recommended damper/duct size
- 12.5 in round
- Nominal stock size
- 13 in
- Rectangular option at 8 in height
- 15.4 x 8 in
Quick answer
Zone dampers are matched to the duct diameter they are installed in, sized so the zone's CFM keeps velocity near normal duct design values.
Damper diameter = duct diameter for the zone's CFM
At a glance
| What it does | Zone damper size for a zoned duct system. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Zone CFM, Design velocity |
| Main output | Recommended damper/duct size |
| Formula | Damper diameter = duct diameter for the zone's CFM |
| Cost | Free — no sign-up, no download |
How to use the Zone Damper Size Calculator
- 1Enter the zone's total design CFM.
- 2Use the normal trunk or branch design velocity for that duct segment.
- 3Order the damper to match the duct size, not a generic default.
Inputs explained
- Zone CFM(CFM)
- Enter the zone cfm in CFM.
- Design velocity(FPM)
- Enter the design velocity in FPM.
Worked example
Using the values the calculator loads with:
Inputs
- Zone CFM600 CFM
- Design velocity700 FPM
Results
- Recommended damper/duct size12.5 in round
- Nominal stock size13 in
- Rectangular option at 8 in height15.4 x 8 in
Frequently asked questions
Should a damper be smaller than the duct?
No, undersized dampers become a permanent restriction; match it to the duct's own sizing.
Motorized or manual?
Zoned systems use motorized dampers tied to a zone control panel; single balancing dampers can be manual.
What do I need to enter into the Zone Damper Size Calculator?
Just 2 values: zone cfm, 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 Zone Damper Size Calculator work out the answer?
Zone dampers are matched to the duct diameter they are installed in, sized so the zone's CFM keeps velocity near normal duct design values. It applies the formula Damper diameter = duct diameter for the zone's CFM and shows the working so you can check each step by hand.
Is the Zone Damper 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.