12" x 6" or 6" x 12" Cube Core Eggcrate Return Air Filter Grille for 1" Filter - Aluminum - White [Outer Dimensions: 14.5" X 8.5"] Curved Blade Steel + Product Details Size -

+ Product Details Size - 20"w X 36"h is the size for the duct opening measurement only

Actual Filter Size – 13 3/4"w x 11 3/4" x 1"

+ Product Details Size – 10"w X 20"h is the size for the duct opening measurement only

actual front end outer dimensions are approximately 26 5/8" width X 10 5/8" height

12" x 6" or 6" x 12" Cube Core Eggcrate Return Air Filter Grille for 1" Filter - Aluminum - White [Outer Dimensions: 14.5" X 8.5"] Curved Blade Steel + Product Details Size -+ Product Details Perfect Fit & Easy Sizing Designed for a 12" x 6" or 6" x 12" duct opening (measured at the opening), with actual outer dimensions of 14. 5" W x 8. 5" H. Please check the measuring guide to ensure a proper fit. Filter Size 11. 75" x 5. 75" x 1" Premium Airflow Efficiency High performance cube core eggcrate design allows 95% airflow efficiency, reducing energy costs and maximizing HVAC system lifespan. Durable, Rust Free Aluminum Made