18" x 36" Cube Core Eggcrate Return Air Filter Grille for 1" Filter 12in diameter + Product Details Size -

+ Product Details Size - 16"w X 12"h is the size for the duct opening measurement only

actual front end outer dimensions are approximately 12 5/8" width X 8 5/8" height

actual front end outer dimensions are approximately 18 5/8" width X 20 5/8" height

+ Product Details Size – 36"w X 10"h is the size for the duct opening measurement only

18" x 36" Cube Core Eggcrate Return Air Filter Grille for 1" Filter 12in diameter + Product Details Size -+ Product Details Perfect Fit & Easy Sizing Designed for a 18" x 36" OR 36" x 18" duct opening (measured at the opening), with actual outer dimensions of 20. 5" W x 38. 5" H. Please check the measuring guide to ensure a proper fit. Filter Size 17. 75" x 35. 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