Damper Sizing Calculator

Damper area from CFM and velocity limits.

Airflow (CFM)

Target velocity (FPM)

System delta P (in. w.g.)

Aspect ratio (W:H)

Blade Type

Tab to move between fields

Damper width (in)

Damper height (in)

Free area (ft²)2.67

Actual velocity (FPM)1,875

Damper drop (in. w.g.)0.22

Damper authority0.31

Authority rating (0-2)1

Equivalent round dia. (in)22.1

Leakage Class I (CFM)10.7

Leakage Class II (CFM)26.7

Required torque (in-lb)19

How to Use This Calculator

How the result changes with Airflow (CFM)

Airflow (CFM): Design airflow through the damper in cubic feet per minute.
Target velocity (FPM): Target face velocity. OA intake: 1000-1500, mixing: 1500-2500, duct: 2000-3000 FPM.
System delta P (in. w.g.): System pressure drop the damper must modulate against (coil, duct, filters). Higher = better authority.
Aspect ratio (W:H): Width-to-height ratio of rectangular damper. 1.0 = square. Keep <=4:1 per AMCA guidelines.
Blade Type: Select damper blade configuration

Damper width (in): Damper width rounded to the nearest 2-inch standard increment.
Damper height (in): Damper height rounded to the nearest 2-inch standard increment.
Free area (ft²): Actual damper free area at the rounded dimensions.
Actual velocity (FPM): Actual face velocity at the selected damper size.
Damper drop (in. w.g.): Pressure drop across the damper when wide open.
Damper authority: Ratio of damper drop to total circuit drop. Target >= 0.5 for good modulating control.
Authority rating (0-2): 0 = Poor (<0.3), 1 = Fair (0.3-0.5), 2 = Good (>=0.5).
Equivalent round dia. (in): Equivalent round duct/damper diameter for the same free area.
Leakage Class I (CFM): Maximum leakage at AMCA Class I (4 CFM/ft²) - low leakage rating.
Leakage Class II (CFM): Maximum leakage at AMCA Class II (10 CFM/ft²) - standard rating.
Required torque (in-lb): Estimated actuator torque requirement based on damper area and blade type.

Identify Input Parameters
4 parameters
Airflow (CFM) = 5000, Target velocity (FPM) = 2000, System delta P (in. w.g.) = 0.5, Aspect ratio (W:H) = 1.5 = 5 input(s) provided
Calculate Damper width
Damper width = ceil(widthIn / 2) * 2
24 = 24
Calculate Damper height
Damper height = ceil(heightIn / 2) * 2
16 = 16
Calculate Free area
Free area = actualAreaSqIn / 144
2.67 = 2.67
Calculate Actual velocity
Actual velocity = cfm / actualAreaSqFt
1875 = 1875