How AC size is worked out
Example: a 14 × 18 ft bedroom is 252 square feet, which the chart puts at 7,000 BTU. It's very sunny, so add 10% to get 7,700, and three people use it, so add 600 for the third person: 8,300 BTU per hour. An 8,000 or 10,000 BTU unit fits.
ENERGY STAR sizing chart
| Room size (sq ft) | Capacity (BTU per hour) |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 300 | 7,000 |
| 300 up to 350 | 8,000 |
| 350 up to 400 | 9,000 |
| 400 up to 450 | 10,000 |
| 450 up to 550 | 12,000 |
| 550 up to 700 | 14,000 |
| 700 up to 1,000 | 18,000 |
| 1,000 up to 1,200 | 21,000 |
| 1,200 up to 1,400 | 23,000 |
| 1,400 up to 1,500 | 24,000 |
| 1,500 up to 2,000 | 30,000 |
| 2,000 up to 2,500 | 34,000 |
The chart assumes 8-foot ceilings. With higher ceilings, a lot of west-facing glass or poor insulation, go a size up. Bigger is not always better: an oversized unit cools the air quickly and shuts off before it removes the humidity, leaving the room cold and clammy and wasting energy.
Frequently asked questions
How many BTUs do I need per square foot?
Roughly 20 BTU per square foot for a typical room, which is where the chart lands for mid-sized rooms: 10,000 BTU covers about 400 to 450 square feet.
What size AC do I need for a 12 × 12 room?
144 square feet calls for about 5,000 BTU, or 5,500 if it's very sunny.
Does a portable AC need more BTUs than a window unit?
Often yes. Portable units lose some efficiency through their hose and the air they pull from the room, so many people choose one size up, and check the newer SACC rating, which reflects real-world cooling better than the older ASHRAE number.
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