Merv 11 Pleated Air Filter - 9 1/2" x 19 1/2" x 1" Quantity:12-pack To find the size the

To find the size the old unit should have sizing information printed on it

In general

with each stage capturing progressively smaller particles inside the filter

if not you can measure it or check the HVAC unit and the owner‰۪s manual

Merv 11 Pleated Air Filter - 9 1/2" x 19 1/2" x 1" Quantity:12-pack To find the size theAny Kind of Filter's Extended Surface Pleated Filters outperform ordinary furnace filters with medium efficiency and are designed for lower pressure drop, and long service life which reduces HVAC equipment maintenance and energy costs. A higher MERV rating means that the air filter that you are about to purchase possesses a higher amount impurity filtration ability than your ordinary filter.