Holmes HWF100 Humidifier Replacement Filter (4 Pack) Improves efficiency of the humidifier Extended filter life with help of reinforce outer layer Extends product life as less mineral deposits will attach to the unit Install at the beginning of each season, replacing every 2 to 4 weeks or as needed Measures approximately 5-3/5 by 4 by 7-4/5 inches- SOLD AS PACK OF 4