AC size for a 400 sq ft room
8,000 BTU
for a 400 sq ft room (0.7 tons) · about 667 W at EER 12
Adjust for the room
| Condition | BTU/h |
|---|---|
| Normal room | 8,000 |
| Very sunny (+10%) | 8,800 |
| Shaded (−10%) | 7,200 |
| Kitchen (+4,000) | 12,000 |
| Each extra person over 2 (+600) | +600 |
Rule of thumb: 20 BTU per sq ft with 8 ft ceilings (ENERGY STAR sizing guidance).
Monthly running cost at $0.17/kWh
| Use | Energy | Cost |
|---|---|---|
| 4 h/day | 80 kWh | $14 |
| 8 h/day | 160 kWh | $27 |
| 12 h/day | 240 kWh | $41 |
FAQ
What size AC for 400 square feet?
About 8,000 BTU (0.7 tons); add 10% for a sunny room and 4,000 BTU for a kitchen.
How much does it cost to run a 8,000 BTU AC?
At roughly 667 W, 8 hours a day for a month uses 160 kWh, about $27 at $0.17/kWh.