What size mini split do I need?

The ENERGY STAR room air conditioner sizing chart puts 450 to 550 sq. ft. at 12,000 BTU/h before any adjustment. On this site that lands in the 12k BTU capacity class. Sun exposure, a kitchen heat load, ceiling height above 8 ft., and insulation quality all move the answer, which is what the calculator above applies.

The same chart puts 700 to 1,000 sq. ft. at 18,000 BTU/h. Above roughly 1,000 sq. ft. a single indoor head is usually being asked to condition more than one room, and airflow between rooms — not capacity — becomes the limit. That is a multi-zone question, and a room-by-room Manual J load calculation is the right tool.

A 20 ft. by 20 ft. garage is 400 sq. ft., which the chart puts at 9,000 BTU/h; a 24 ft. by 24 ft. garage is 576 sq. ft., which the chart puts at 14,000 BTU/h. Treat both as a floor rather than an answer. The ENERGY STAR chart describes conditioned rooms, and most garages have an uninsulated door, little or no wall insulation, and a high ceiling — all of which push the real load up. Have an installer run a load calculation before buying for a garage.

There is no single figure. Across the ENERGY STAR chart the implied rate runs from about 33 BTU/h per square foot in the smallest band down to about 14 BTU/h per square foot in the largest. Larger rooms need fewer BTU per square foot, so a flat per-square-foot rule oversizes small rooms and undersizes large ones.

Yes. The chart is written for an 8 ft. ceiling, so the calculator above scales the floor area by your ceiling height divided by 8 before reading the chart. A 400 sq. ft. room with a 12 ft. ceiling is treated as 600 sq. ft. of load.

No. The chart is a screening estimate from floor area. ACCA Manual J is the residential load calculation standard and accounts for construction, orientation, window area, infiltration, and local design conditions. Use this page to get to the right size class and to shop; use Manual J before you buy.