The short answer
A room air conditioner needs about 20 BTU per square foot. Using the ENERGY STAR sizing chart, a 300 square foot room needs 8,000 BTU, a 450 square foot room needs 12,000 BTU, and a 1,000 square foot space needs 21,000 BTU. Add 10% for a very sunny room, 600 BTU for each person beyond two, and 4,000 BTU for a kitchen.
How the calculation works
For cooling, the calculator uses the ENERGY STAR room air conditioner sizing chart and its published adjustments:
- Look up the BTU for the room’s floor area (the chart assumes 8-foot ceilings).
- Scale for ceiling height: a 10-foot ceiling adds 25%.
- Heavily shaded room: subtract 10%. Very sunny room: add 10%.
- Add 600 BTU for each regular occupant beyond two.
- Add 4,000 BTU if the unit is for a kitchen.
For heating, it multiplies the floor area by a rule-of-thumb figure for your climate, from 30 BTU per square foot in hot climates to 55 in cold ones.
Worked example
A sunny 20 × 15 ft living room with 8-foot ceilings, regularly used by four people:
- 300 square feet → 8,000 BTU on the chart
- Very sunny: 8,000 × 1.10 = 8,800 BTU
- Two extra people: + 1,200 BTU
- Total: 10,000 BTU, so a 10,000 BTU unit is the right size
ENERGY STAR sizing chart
| Room area | Cooling capacity |
|---|---|
| 100 to 150 sq ft | 5,000 BTU |
| 150 to 250 sq ft | 6,000 BTU |
| 250 to 300 sq ft | 7,000 BTU |
| 300 to 350 sq ft | 8,000 BTU |
| 350 to 400 sq ft | 9,000 BTU |
| 400 to 450 sq ft | 10,000 BTU |
| 450 to 550 sq ft | 12,000 BTU |
| 550 to 700 sq ft | 14,000 BTU |
| 700 to 1,000 sq ft | 18,000 BTU |
| 1,000 to 1,200 sq ft | 21,000 BTU |
| 1,200 to 1,400 sq ft | 23,000 BTU |
| 1,400 to 1,500 sq ft | 24,000 BTU |
| 1,500 to 2,000 sq ft | 30,000 BTU |
| 2,000 to 2,500 sq ft | 34,000 BTU |
Heating rule of thumb
| Climate | BTU per sq ft | 300 sq ft room |
|---|---|---|
| Hot | 30 | 9,000 BTU |
| Warm | 35 | 10,500 BTU |
| Mixed | 40 | 12,000 BTU |
| Cool | 45 | 13,500 BTU |
| Cold | 55 | 16,500 BTU |
When to get a professional load calculation
Room rules of thumb are fine for window units, portable ACs and space heaters. For central air, heat pumps or a furnace, ask for a Manual J load calculation, which accounts for insulation, windows, air leakage and orientation room by room. Oversized whole-house systems are common and costly.
Frequently asked questions
How many BTU do I need per square foot?
Roughly 20 BTU per square foot for cooling with 8-foot ceilings. The ENERGY STAR chart gives more per square foot for small rooms (5,000 BTU covers 100 to 150 square feet) and less for large open spaces, about 14 to 18 BTU per square foot above 1,000 square feet.
What size AC do I need for a 12 × 12 room?
A 12 × 12 ft room is 144 square feet, which calls for a 5,000 BTU unit on the ENERGY STAR chart. Step up to 6,000 BTU if the room is very sunny or used by more than two people.
Is a bigger air conditioner better?
No. An oversized unit cools the room quickly and shuts off before removing humidity, leaving the room cold and clammy. It also cycles on and off more, which wastes energy.
How many square feet will a 1-ton AC cool?
One ton is 12,000 BTU per hour, which the ENERGY STAR chart matches to about 450 to 550 square feet for a single room.
How many BTU do I need to heat a room?
As a rule of thumb, 30 BTU per square foot in hot climates up to 55 or more in cold climates. Insulation and windows matter a lot, so treat this as a rough guide.
Sources
Related calculators
About this calculator. Every formula on this page is tested against worked examples from the sources above before publishing. Results are estimates for planning; confirm with your supplier or local code.
Spotted a problem? Tell us and we will check it.