17.5X17.5X.75 actual size (18 x 18 x 1 nominal size). The 18x18x1 air filter is an extended surface pleated filter, which outperforms standard air filters with medium efficiency, and are built for lower pressure drop, and longer service life, which reduces HVAC equipment maintenance and energy expenses. This 18x18x1 air filter is graded to ASHRAE test standards 52.2, which ensures that the air filter will function as good or better than OEM 18x18x1 air filters. Pleated design. MERV tested to ASHRAE grades. 664 cfm flow rate.