10" x 26" or 26" x 10" Cube Core Eggcrate Return Air Filter Grille for 1" Filter - Aluminum - White [Outer Dimensions: 12.5" X 28.5"] Grilles + Product Details Size -

+ Product Details Size - 36"w X 4"h is the size for the duct opening measurement only

actual front end outer dimensions are approximately 30 5/8" width X 30 5/8" height

Crafted from durable ABS plastic

actual front end outer dimensions are approximately 12 5/8" width X 34 5/8" height

+ Product Details Size - 26"w X 20"h is the size for the duct opening measurement only

10" x 26" or 26" x 10" Cube Core Eggcrate Return Air Filter Grille for 1" Filter - Aluminum - White [Outer Dimensions: 12.5" X 28.5"] Grilles + Product Details Size -+ Product Details Perfect Fit & Easy Sizing Designed for a 10" x 26" or 26" x 10" duct opening (measured at the opening), with actual outer dimensions of 12. 5" W x 28. 5" H. Please check the measuring guide to ensure a proper fit. Filter Size 9. 75" x 25. 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