21 In.H-W 4.5 In.L Aluminum Backdraft Damper, Damper Application Backdraft, For Nominal Duct Diameter Not Applicable, Frame Material Galvanized Steel, Number of Panels 1, Maximum Air Velocity 2000 fpm, Overall Height 21 in, Rough Opening Diameter 19 1/2 in, Rough Opening Shape Square, Fits Fan Diameter 18 in, For Duct Shape Square, Free Area 1.9 sq ft, Overall Width 21 in, Frame Material Thickness 20 ga, Overall Length 4 1/2 in, Frame Depth 2 in, Rough Opening Length 18 1/2 in