12" X 8" ADJUSTABLE AIR SUPPLY DIFFUSER - HVAC Vent Duct Cover Sidewall or Ceiling 12" Diameter then you'll need a 14-inch

then you'll need a 14-inch round vent

+ Product Details Size – 28"w X 30"h is the size for the duct opening measurement only

Actual Filter Size – 7 3/4"w x 19 3/4" x 1"

Actual Filter Size – 11 3/4"w x 27 3/4" x 1"

Weather and Pest Proof – These louvers are especially designed to prevent water from entering your home

12" X 8" ADJUSTABLE AIR SUPPLY DIFFUSER - HVAC Vent Duct Cover Sidewall or Ceiling 12" Diameter then you'll need a 14-inch+ Product Details Size 12"w X 8"h is the size for the duct opening measurement only, actual front end outer dimensions are approximately 13. 875" width X 10" height. High Quality High quality steel construction, no scuff powder coat durable finish, smooth, shiny, scratch resistant, no sharp edges, 2 recessed screw holes on each side with screws included, screws same color as vent cover, easy installation. Adjustable airflow control Adjustable airflow