Aerostar 21x21x1 MERV 11 - 6 Count - Pleated HVAC, Furnace & Air Conditioner Filters - Removes Dust, Cold & Flu Germs, Lint, Mold, Pet Dander, Allergy-Causing Pollen, Smog & SmokeNominal size: 21x21x1; Actual size: 20. 875" x 20. 875" x 0. 75"; This MERV 11 filter is comparable with MPR 1000 1200 & FPR 7 Designed to filter microscopic particles such as lint, dust mites, spores, pollen, pet dander, fine dust, and smoke Simple to install and a cost effective step towards a healthier home environment. For best results, change your filters every 60 to 90 days but increase frequency to 30 days during peak summer and winter months