36" x 18" Cube Core Eggcrate Return Air Filter Grille for 1" Filter w10h10 actual front end outer dimensions

actual front end outer dimensions are approximately 32 5/8" width X 12 5/8" height -

Think of the diameter as a straight line from one end to the other

+ Product Details Size - 20"w X 8"h is the size for the duct opening measurement only

actual front end outer dimensions are approximately 27 5/8" width X 27 5/8" height

36" x 18" Cube Core Eggcrate Return Air Filter Grille for 1" Filter w10h10 actual front end outer dimensions+ Product Details Perfect Fit & Easy Sizing Designed for a 36" x 18" OR 18" x 36" duct opening (measured at the opening), with actual outer dimensions of 38. 5" W x 20. 5" H. Please check the measuring guide to ensure a proper fit. Filter Size 35. 75" x 17. 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