Aerostar 21x21x1 MERV 11 Pleated Air Filter, AC Furnace Air Filter, 6 Pack (Actual Size: 20.875 x 20.875 x 0.75) - Nominal 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 - 100% synthetic electrostatic media for high efficiency with low air flow resistance - Manufactured in the USA by Filtration Group, a world-leading manufacturer and supplier of filtration solutions making the world safer, healthier & more productive