18" x 20" or 20" x 18" Cube Core Eggcrate Return Air Filter Grille for 1" Filter 18" Round Duct Pipe + Product Details Size -

+ Product Details Size - 4"w X 28"h is the size for the duct opening measurement only

Ring Base Design – Allows for easy installation without removing the thermostat

Its durability and performance contribute to overall comfort

Universal Fit – Dimensions of 6-3/8" wide x 3-1/2" high x 3" deep make it compatible with most standard thermostat models

18" x 20" or 20" x 18" Cube Core Eggcrate Return Air Filter Grille for 1" Filter 18" Round Duct Pipe + Product Details Size -+ Product Details Perfect Fit & Easy Sizing Designed for a 18" x 20" OR 20" x 18" duct opening (measured at the opening), with actual outer dimensions of 20. 5" W x 22. 5" H. Please check the measuring guide to ensure a proper fit. Filter Size 17. 75" x 19. 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