Accumulair Diamond 18x20x4 (Actual Size) MERV 13 Air Filter/Furnace Filters (2 pack) Actual Size of Filter: 18 x 20 x 3.75 MERV 13 Rating. Each filter lasts up to 3 months. Uniform pleat shape enhances dust holding capacity and maximizes air flow. Supercharged pleated panel filtering media ideally suited for capturing fine airborne allergens such as dust, smoke, dander, mold, pet dander, and dust mites. 2 Pack of Air Filters