16x18x1 MERV 2 Non Pleated Air Filter Replacement Filters (Actual Size 15.75 x 17.75) - 3 Pack 5" Round Duct Pipe then you'll need a 14-inch

then you'll need a 14-inch round vent

diagonally feed it through the square hole

+ Product Details Perfect Fit & Easy Sizing - Designed for a 30" x 20" OR 20" x 30" duct opening (measured at the opening)

Actual Filter Size – 35 3/4"w x 24 3/4" x 1"

16x18x1 MERV 2 Non Pleated Air Filter Replacement Filters (Actual Size 15.75 x 17.75) - 3 Pack 5" Round Duct Pipe then you'll need a 14-inch+ Product Details 16" x 18" x 1" replacement air filters for your furnace, air conditioner, heat pump, or HVAC system (actual size: 15. 75" x 17. 75") A unique combination of cotton & synthetic fibers gives you the most filtration and the least air resistance equals great air & easy airflow for your system. The non pleated design provides low airflow resistance while still effectively capturing dirt and particles such as standing dust, spray paint