While there is no exact formula to choose the right subwoofer size for a given room, we generally follow some conventions and proceed to advise more meticulously based on individual requirements. We generally recommend up to a 12” sub for rooms with an area of 150 sq. ft. or less. 15” subs can be used in rooms up to 250 sq. ft. whereas 18” subs are a good fit for rooms up to 400 sq. ft in area. 21” subs and larger are generally recommended for larger rooms.