21x26x5 (20.1x25.7x5) MERV 11 Aftermarket American Standard Replacement Filter (2 Pack) The nominal size is 21 x 26 x 5. The actual size is 20.1 x 25.7 x 5. It has 98% efficient filtration media. It is up to twenty times more effective than ordinary fiberglass filters. High-efficiency filter media collects small dust particles before they can collect on your coils. This is an aftermarket filter.