This Radio Shack humidifier wick replacement filter fits Radio Shack 63-1508 and HCM551. It measures approximately 4 5/8 H x 5 ID x 6 1/2 OD x 3/4 Thick and prevents surface growth and migration of mold, fungus, algea, and bacteria. For optimum performance, replace filter at the beginning of each season or every 2 months depending on use.