How many BTU do I need?
For cooling, ENERGY STAR's sizing chart works out to roughly 20 BTU per square foot for small rooms, falling to about 14 for large spaces. Then adjust:
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase by 10%.
- More than two people regularly: add 600 BTU per extra person.
- Kitchen: add 4,000 BTU.
- Ceilings above 8 ft: scale up in proportion (a 10 ft ceiling ≈ +25%).
Air conditioner BTU chart (ENERGY STAR)
| Room size | Cooling capacity |
|---|---|
| 100–150 sq ft | 5,000 BTU/h |
| 150–250 sq ft | 6,000 BTU/h |
| 250–300 sq ft | 7,000 BTU/h |
| 300–350 sq ft | 8,000 BTU/h |
| 350–400 sq ft | 9,000 BTU/h |
| 400–450 sq ft | 10,000 BTU/h |
| 450–550 sq ft | 12,000 BTU/h |
| 550–700 sq ft | 14,000 BTU/h |
| 700–1,000 sq ft | 18,000 BTU/h |
| 1,000–1,200 sq ft | 21,000 BTU/h |
| 1,200–1,400 sq ft | 23,000 BTU/h |
| 1,400–1,500 sq ft | 24,000 BTU/h |
| 1,500–2,000 sq ft | 30,000 BTU/h |
| 2,000–2,500 sq ft | 34,000 BTU/h |
Heating BTU by climate zone
A common rule of thumb for heat loss with average insulation and 8 ft ceilings:
| Climate zone | BTU per sq ft | 400 sq ft room |
|---|---|---|
| Zone 1 | 30 | 12,000 BTU/h |
| Zone 2 | 35 | 14,000 BTU/h |
| Zone 3 | 40 | 16,000 BTU/h |
| Zone 4 | 45 | 18,000 BTU/h |
| Zone 5 | 50 | 20,000 BTU/h |
| Zone 6 | 55 | 22,000 BTU/h |
| Zone 7 | 60 | 24,000 BTU/h |
Electric heaters: 1 watt = 3.412 BTU/h, so a 1,500 W space heater puts out about 5,100 BTU/h — enough for roughly 100–150 sq ft in a cold climate.
FAQ
How many square feet does a 12,000 BTU AC cool?
About 450–550 sq ft per the ENERGY STAR chart — 12,000 BTU is also "1 ton" of cooling.
Is a bigger air conditioner better?
No. An oversized unit cools the air quickly and shuts off before it removes humidity, leaving the room cold and clammy and wasting energy.
Can I use this for central air or a furnace?
Only as a ballpark. Whole-house systems should be sized with an ACCA Manual J load calculation, which accounts for windows, insulation, air leakage and orientation.