Accumulair 21x21.5x5 (20.1x21.1x5) MERV 8 Aftermarket Trane Replacement Filter Nominal size 21x21.5x5 actual size 20.1x21.1x5 20 times more efficient than ordinary fiberglass filters Electrostatically charged filter media captures airborne allergens smaller than 1 micron Lasts up to 3 months Merv rating: 8