12" x 18" or 18" x 12" Cube Core Eggcrate Return Air Filter Grille for 1" Filter - Aluminum - White [Outer Dimensions: 14.5" X 20.5"] 10in diameter Ideal for Walls & Ceilings:

Ideal for Walls & Ceilings: Designed for seamless installation in standard 16" X 16" duct openings

The first number in a grille size represents the width

actual front end outer dimensions are approximately 20 5/8" width X 22 5/8" height

Actual Filter Size – 29 3/4"w x 9 3/4" x 1"

12" x 18" or 18" x 12" Cube Core Eggcrate Return Air Filter Grille for 1" Filter - Aluminum - White [Outer Dimensions: 14.5" X 20.5"] 10in diameter Ideal for Walls & Ceilings:+ Product Details Perfect Fit & Easy Sizing Designed for a 12" x 18" or 18" x 12" duct opening (measured at the opening), with actual outer dimensions of 14. 5" W x 20. 5" H. Please check the measuring guide to ensure a proper fit. Filter Size 11. 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