Formula used
Required effective area: Ae = Q ÷ V
Required gross area: Ag = Ae ÷ free-area fraction
Rectangular area: A = W × H
Round area: A = πD² ÷ 4
Free-area velocity: Vf = Q ÷ Ae
Pressure drop: ΔP = K × ½ρVf²
Equal-area diameter: D = √(4A ÷ π)
Air density: ρ = P ÷ RT, using calculated site pressure.
These formulas support preliminary HVAC sizing. Manufacturer coefficients provide better final accuracy. Certified selections must follow project specifications and applicable codes.
How to use the calculator
- Select the calculation mode and damper shape.
- Choose a damper and blade type.
- Enter airflow, velocity, and available dimensions.
- Set free area, margin, and pressure limits.
- Choose automatic or manual air density.
- Set standard increments and section limits.
- Enter torque and leakage assumptions.
- Press calculate and review every warning.
- Compare standard sizes before final selection.
- Export results for project documentation.
Example damper sizing data
| Example | Airflow | Allowable velocity | Free area | Suggested use |
|---|---|---|---|---|
| Quiet office supply | 2,500 CFM | 700 FPM | 65% | Low-noise control damper |
| General AHU branch | 5,000 CFM | 1,000 FPM | 65% | Volume control damper |
| Industrial exhaust | 12,000 CFM | 1,500 FPM | 55% | Heavy-duty exhaust damper |
| Round duct branch | 1,500 CFM | 900 FPM | 70% | Round butterfly damper |
Damper selection guidance
Gross area is the full frame opening. Free area excludes blades, frames, and internal obstructions. Lower free area increases velocity and pressure loss.
Opposed blades usually provide smoother throttling. Parallel blades can provide rapid mixing and different control behaviour. Actual performance depends on the manufacturer’s tested geometry.
Keep aspect ratios practical for stable construction. Large units often need multiple sections. Blade length and actuator linkage can limit single-section sizes.
Fire and smoke dampers are life-safety products. Do not size them using airflow alone. Verify listings, sleeves, retaining angles, access, clearances, and installation instructions.
Frequently asked questions
What velocity should I use?
Use the project’s acoustic, pressure-loss, and control requirements. Lower velocities usually reduce noise and resistance.
Why does free area matter?
Air accelerates through the unobstructed opening. Pressure loss depends strongly on this higher free-area velocity.
Should I size from face velocity or free-area velocity?
Use the basis stated by the manufacturer or specification. This calculator reports both for comparison.
How is pressure drop estimated?
The calculator uses a K-factor relationship. Tested manufacturer curves remain the preferred final source.
What is a good aspect ratio?
Ratios near one are structurally convenient. Very wide or tall dampers may require additional sections.
When is a multi-section damper needed?
Use sections when dimensions exceed blade, frame, shipping, linkage, or manufacturer module limits.
Can this calculator select an actuator?
It provides a rough torque estimate. Add manufacturer torque data, linkage losses, seals, and safety factors.
How is leakage estimated?
Leakage scales from the entered area rate and pressure reference. Use certified leakage data when available.
Does altitude affect sizing?
Altitude changes air density and pressure loss. Volumetric airflow area remains governed mainly by airflow and velocity.
Can it size fire or smoke dampers?
It can estimate airflow area only. Final life-safety selection requires approved listings and code compliance.
Why are standard sizes rounded upward?
Rounding upward protects airflow capacity and reduces velocity. The comparison table shows nearby practical alternatives.
Engineering disclaimer
This calculator supports preliminary design and education. It does not replace certified damper data, licensed engineering judgement, project specifications, testing standards, or local regulations.