18" x 30" x 1", red, micro allergen filtrete pleated filter, 3 month filter is 90% effective at capturing large allergens like mold spores & pet dander, electrostatically charged, captures microscopic allergens like dust, smoke & smog, captures bacteria & particles that can carry viruses, meets the guidelines of the american lung association health house program, mpr rating 1000, lasts up to 3 months.