Enter the size of the room and a few details about it. The calculator gives the cooling BTUs you need, the heating BTUs for your climate, and the standard air conditioner size to buy.
BTUs you need
Standard air conditioner sizes (the size to buy is highlighted)
| AC size | Cools up to | Tons |
|---|
How many BTUs do I need per square foot?
For cooling, the starting point is about 20 BTU per hour for each square foot of living space. That is the basis of the ENERGY STAR room air conditioner chart the calculator uses. The chart is then adjusted for your room:
- Sun: add 10% for a very sunny room, take off 10% for a heavily shaded one.
- People: add 600 BTU for every regular occupant beyond two.
- Kitchen: add 4,000 BTU, because cooking appliances put a lot of heat into the room.
- Ceiling height: the chart assumes 8 ft ceilings. Higher ceilings mean more air to cool, so the result scales up.
| Room area | Cooling capacity needed |
|---|---|
| 100–150 sq ft | 5,000 BTU |
| 150–250 sq ft | 6,000 BTU |
| 250–300 sq ft | 7,000 BTU |
| 300–350 sq ft | 8,000 BTU |
| 350–400 sq ft | 9,000 BTU |
| 400–450 sq ft | 10,000 BTU |
| 450–550 sq ft | 12,000 BTU |
| 550–700 sq ft | 14,000 BTU |
| 700–1,000 sq ft | 18,000 BTU |
| 1,000–1,200 sq ft | 21,000 BTU |
| 1,200–1,400 sq ft | 23,000 BTU |
| 1,400–1,500 sq ft | 24,000 BTU |
| 1,500–2,000 sq ft | 30,000 BTU |
| 2,000–2,500 sq ft | 34,000 BTU |
Heating BTUs
Heating needs depend much more on climate than cooling does. A room in Florida may need 25 BTU per square foot on the coldest night, while the same room in Minnesota can need 60 or more. Pick your climate in the calculator to see the difference.
What happens if the AC is too big or too small?
An undersized unit runs constantly and never reaches the set temperature on hot afternoons. An oversized unit is just as bad in a different way: it cools the air quickly and shuts off before it has removed much humidity, so the room feels cold and clammy, and the short on-off cycles wear the compressor. Buy the size the calculator highlights, not the next one up “to be safe”.
Tons and BTUs
One ton of cooling equals 12,000 BTU per hour. A 24,000 BTU unit is a 2-ton unit. Whole-house systems are sold in half-ton steps from 1.5 to 5 tons; for those, use the heat pump size calculator.