What size air conditioner for a 12x12 room?
Quick answer: A 12 x 12 room is 144 sq ft, inside ENERGY STAR's 100 to 150 sq ft row: a 5,000 BTU/h room air conditioner, assuming 8 ft ceilings. ENERGY STAR says to cut 10% for a heavily shaded room, add 10% for a very sunny one and add 600 BTU/h per person beyond two.
Your room, your adjustments
ENERGY STAR room AC sizing table
| Area to be cooled | Capacity needed |
|---|---|
| 100 up to 150 sq ft | 5,000 BTU/h |
| 150 up to 250 sq ft | 6,000 BTU/h |
| 250 up to 300 sq ft | 7,000 BTU/h |
| 300 up to 350 sq ft | 8,000 BTU/h |
| 350 up to 400 sq ft | 9,000 BTU/h |
| 400 up to 450 sq ft | 10,000 BTU/h |
| 450 up to 550 sq ft | 12,000 BTU/h |
| 550 up to 700 sq ft | 14,000 BTU/h |
| 700 up to 1,000 sq ft | 18,000 BTU/h |
| 1,000 up to 1,200 sq ft | 21,000 BTU/h |
| 1,200 up to 1,400 sq ft | 23,000 BTU/h |
| 1,400 up to 1,500 sq ft | 24,000 BTU/h |
| 1,500 up to 2,000 sq ft | 30,000 BTU/h |
| 2,000 up to 2,500 sq ft | 34,000 BTU/h |
Source: ENERGY STAR Room Air Conditioners (8 ft ceilings). Adjustments from the same page. Machine-readable copy: ENERGY STAR sizing data.
About the 144 sq ft answer
12 ft × 12 ft = 144 sq ft, which is under the 150 sq ft line, so the chart starts at 5,000 BTU/h. A very sunny room adds 10% (5,500 BTU/h), and a third person adds 600 BTU/h. Small rooms on the edge of a row are covered in more detail on the 150 sq ft page. This is a room-unit chart, not a whole-house load calculation.