Air conditioner size calculator
What size window or room air conditioner a room needs, in BTU, using the ENERGY STAR sizing chart.
Air conditioner size
How to use it
Enter the room's length and width in feet and how many people regularly use it. Choose how sunny it is, and tick the box if it's a kitchen. The calculator gives the cooling capacity to look for, in BTU per hour, the number printed on every room air conditioner.
This calculator covers rooms from 100 to 400 sq ft with 8 ft ceilings, the range window and portable units are made for. For larger spaces or high ceilings, a contractor's load calculation gives a far better answer.
How it's calculated
ENERGY STAR's chart sets the starting size by floor area:
| Room area | 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 |
Then adjust:
- Heavily shaded room: reduce by 10%.
- Very sunny room: increase by 10%.
- More than two people: add 600 BTU for each extra person.
- Kitchen: add 4,000 BTU.
A shaded 120 sq ft kitchen, for example: 5,000 × 0.9 + 4,000 = 8,500 BTU.
Tips
- Bigger isn't better. An oversized unit cools the room fast but switches off before it removes humidity, leaving the room cold and clammy.
- Measure the window before you buy: units list the window width and height they fit.
- Check the plug. Larger units can need a dedicated circuit or a 230 V outlet.