36" x 18" Cube Core Eggcrate Return Air Filter Grille for 1" Filter w10h10 Think of the diameter as

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

+ Product Details Perfect Fit & Easy Sizing - Designed for a 32" x 36" Or 36" x 32" duct opening (measured at the opening)

+ Product Details Size - 6"w X 20"h is the size for the duct opening measurement only

+ Product Details Size – 25"w X 24"h is the size for the duct opening measurement only

+ Product Details Size – 16"w X 36"h is the size for the duct opening measurement only

36" x 18" Cube Core Eggcrate Return Air Filter Grille for 1" Filter w10h10 Think of the diameter as+ 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