Filter King Merv 8 18x20x1 12-Pack Air Filter actual size is 17.5x19.5x.75. Our 18x20x1 Furnace Filters are designed to last up to 90 days. These pleated 1-inch air filters are ideal for maintaining the efficiency of your heating and cooling systems. Make sure you measure your filter. The actual size of this pleated Merv 8 filter is 17-1/2 x 19-1/2 x 3/4