Proper sizing of a ductless mini-split system depends on the room’s square footage, insulation quality, and climate. General guidelines suggest 9,000 BTU for 250-350 sq. ft., 12,000 BTU for 350-500 sq. ft., and 18,000 BTU for 500-750 sq. ft. However, high ceilings, sun exposure, and insulation quality can affect sizing. A professional load calculation ensures the correct BTU capacity for maximum efficiency.