The Atomic FC100A1011 20"x20" Honeywell replacement media AC Filter is a high-efficiency Media Air Filter that can be used in Honeywell 20 x 20 Air Cleaners. The nominal size may be listed as 20x20x4 or 20x20x5 This is a genuine Atomic replacement for a Honeywell air conditioning filter.Honeywell FC100A1011 Specifications: Nominal Size: 20" x 20" x 4 3/8" Actual Size: 19 7/8" x 19 7/8" x 4 3/8" MERV 11 Rating Replaces FC35A1019 / 203721 Fits Honeywell Series: F25, F35, F100, and F150. Atomic FC100A1011 is the perfect replacement for your Honeywell High-Efficiency Media Air Cleaners. This air conditioner filter should be replaced at least once per year for maximum efficiency. Filter life based on air quality.