30"w X 8"h Aluminum Exterior Gable Louver Vent for Walls & Crawlspace - Weather-Resistant Air Grille with Screen Mesh - [Outer Dimensions 31.75"w x 9.75"h] Aluminum Adjustable Curved Blade + Product Details Size -

+ Product Details Size - 26"w X 36"h is the size for the duct opening measurement only

Actual Filter Size – 33 3/4"w x 15 3/4" x 1"

you can attain the diameter of the vent

actual front end outer dimensions are approximately 20 5/8" width X 24 5/8" height

Actual Filter Size – 17 3/4"w x 23 3/4" x 1"

30"w X 8"h Aluminum Exterior Gable Louver Vent for Walls & Crawlspace - Weather-Resistant Air Grille with Screen Mesh - [Outer Dimensions 31.75"w x 9.75"h] Aluminum Adjustable Curved Blade + Product Details Size -+ Product Details Size 30"w X 8"h X 2"d (for duct opening measurement only); actual front end outer dimensions are 31. 75" Width X 9. 75" Height Weather and Pest Proof These louvers are especially designed to prevent water from entering your home, and with a built in back 1 2" mesh they also protect you from annoying rodents like mice, rats, birds, and squirrels High Quality Aluminum The premium durable aluminum will never rust and it will never wear