10" x 8" or 8" x 10" Cube Core Eggcrate Return Air Filter Grille for 1" Filter - Aluminum - White [Outer Dimensions: 12.5" X 10.5"] return filter + Product Details Size –

+ Product Details Size – 20"w X 18"h is the size for the duct opening measurement only

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but still very thin

10"w X 3"h Is The Size For The Duct Opening Measurement Only

10" x 8" or 8" x 10" Cube Core Eggcrate Return Air Filter Grille for 1" Filter - Aluminum - White [Outer Dimensions: 12.5" X 10.5"] return filter + Product Details Size –+ Product Details Perfect Fit & Easy Sizing Designed for a 10" x 8" or 8" x 10" duct opening (measured at the opening), with actual outer dimensions of 12. 5" W x 10. 5" H. Please check the measuring guide to ensure a proper fit. Filter Size 9. 75" x 7. 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