HVAC Plenum Ceiling Box 14X14X10 (14"X14" Square to 10" Round) with Drywall Flange - Connects to Vent Register Diffuser, Galvanized Steel Metal C-Box Duct, Airflow Conditioning Ceiling Registers;

14 Inches X 14 Inches is the actual size of the square portion. Your diffuser or register will be slightly larger than this.