16" X 26" Steel Return Air Filter Grille for 1" Filter - Removable Frame - [Outer Dimensions: 18 5/8" X 28 5/8"] Supply Grille then you'll need a 14-inch

then you'll need a 14-inch round vent

+ Product Details Perfect Fit & Easy Sizing - Designed for a 14" x 20" OR 20" X 14" duct opening (measured at the opening)

Actual Filter Size – 9 3/4"w x 35 3/4" x 1"

and the second number represents the height

16" X 26" Steel Return Air Filter Grille for 1" Filter - Removable Frame - [Outer Dimensions: 18 5/8" X 28 5/8"] Supply Grille then you'll need a 14-inch+ Product Details Size 16"w X 26"h is the size for the duct opening measurement only, actual front end outer dimensions are approximately 18 5 8" width X 28 5 8" height Actual Filter Size 15 3 4"w x 25 3 4" x 1" High Quality High quality solid stamped steel with counter sunk screw holes (screws included) and no sharp edges Installation Location Sidewall and ceiling Design Removable door face for easy filter change & cleaning uses a 1" thick filter.