Damper Sizing Calculator

Inputs

Use the grid below; it adapts to screen size.

Run once to enable downloads.

Display system

Affects recommended sizes and constraints display.

Damper shape

Choose what you plan to install.

Sizing basis

Free-area basis is more conservative for blade losses.

Airflow

Enter total airflow through the damper.

Velocity limit

Typical comfort HVAC dampers often target 500–1200 fpm.

Free area ratio

0–1

Net open area / face area. Default: 0.85

Safety factor

%

Extra face area to allow future balancing. Default: 10%

Loss coefficient K

Used for ΔP estimate. Typical range: 1–6.

Temperature

°C

Impacts density and pressure estimate.

Altitude

m

Higher altitudes reduce density and ΔP.

Standard size range (min)

Use inches or millimeters, per display system.

Standard size range (max)

Increase max to search larger options.

Increment

Common increments are 2 in or 50 mm.

Target aspect ratio (W:H)

Only used for rectangular selection preference.

Max width / diameter (optional)

Limit width or diameter when space is tight.

Max height (optional)

Only affects rectangular selections.

Reset

Example data table

Use these as starting points for typical duct branches.

Scenario	Airflow	Velocity limit	Shape	Suggested size	Estimated ΔP
Office supply branch	1500 CFM	900 fpm	Rectangular	18×14 in	~35 Pa
Return air riser	2500 CFM	1100 fpm	Rectangular	22×16 in	~55 Pa
Round duct takeoff	700 CFM	800 fpm	Round	Ø 12 in	~22 Pa

Values are illustrative; your project conditions may differ.

Formula used

Q = airflow, V = velocity limit.
Face area (if sizing by face velocity): A_face = Q / V_face
Free area ratio: r = A_free / A_face
Free area: A_free = A_face × r
If sizing by free-area velocity: A_free = Q / V_free then A_face = A_free / r
Safety factor: A_target = A_face × (1 + SF/100)
Pressure estimate (using free-area velocity): ΔP = K × ½ × ρ × V_free²

This tool estimates a practical nominal size based on standard increments and your constraints.

How to use this calculator

Enter airflow for the branch or section being controlled.
Pick a velocity limit suitable for noise and pressure goals.
Choose damper shape and the sizing basis you prefer.
Adjust free area ratio and safety factor if required.
Optionally set max width/height to match available space.
Press Calculate to view results above the form.
Use CSV or PDF buttons to export the calculation.

For final selections, confirm manufacturer free area and published pressure data.

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