17.5X23.5X.75 actual size (18 x 24 x 1 nominal size). The 18x24x1 air filter is an extended surface pleated filter, which outperforms standard air filters with medium efficiency, and are built for lower pressure drop, and long service life, which reduces HVAC equipment maintenance and energy expenses. This 18x24x1 air filter is measured to ASHRAE test standards 52.2, which establishes that the air filter will filter as good or better than original brand 18x24x1 air filters. Pleated design. MERV classified to ASHRAE ratings. 885 cfm flow rate.