Replace every 6 months if you have heating & cooling or more often if you have indoor pets. True Exact Dimensions are 15 3/4"x 24 3/4 "x 4 3/8"- to replace Honeywell FC100A1029 and other brands with the same dimensions. Note: Many "5" filter frames actually require 4 1/2" thick filters- measure before ordering. High Efficiency Merv 11 model Replaces #11-GA100A1029- Pack of 2 Filters