Picking an air conditioner starts with one number: BTUs. It is the measure of how much heat a unit can pull out of a room per hour, and getting it right is the difference between a room that cools evenly and one that never quite feels comfortable. Too small and the unit runs nonstop without catching up. Too large and it cools so fast it shuts off before it can pull the humidity out, leaving the room cold and clammy.
The good news is that sizing is mostly about your square footage, with a few easy adjustments. Here is the chart, followed by the tweaks that matter.
BTU by room size chart
These figures follow the ENERGY STAR and EPA sizing guidance for a standard room with 8 ft ceilings.
| Room size (sq ft) | Cooling capacity (BTU/hr) | Typical space |
|---|---|---|
| 100 to 250 | 5,000 to 6,000 | Small bedroom, office, nursery |
| 250 to 350 | 7,000 to 8,000 | Bedroom, home office |
| 350 to 450 | 9,000 to 10,000 | Large bedroom, den |
| 450 to 550 | 12,000 | Studio, small living room |
| 550 to 700 | 14,000 | Living room, open kitchen |
| 700 to 1,000 | 18,000 | Large living room, small apartment |
| 1,000 to 1,400 | 21,000 to 24,000 | Open-plan main floor |
How to measure your room
Multiply length by width to get square footage. For an L-shaped or open-plan space, break it into rectangles, measure each, and add them together. If the air conditioner has to cool through a doorway into a second room, count that space too.
Adjustments that change your BTU
The chart is a starting point. Nudge it up or down for real-world conditions:
- Very sunny room: add about 10 percent.
- Heavily shaded room: subtract about 10 percent.
- More than two people regularly in the room: add 600 BTU per extra person.
- Kitchen: add about 4,000 BTU for the heat from cooking.
- Ceilings above 8 ft: size up one step, since you are cooling more air volume.
Why bigger is not better
It is tempting to buy the largest unit you can afford, but an oversized air conditioner is a common and costly mistake. It cools the air quickly, hits the thermostat target, and shuts off before it has run long enough to remove humidity. The result is short, frequent cycles that leave the room cold and damp, wear the compressor faster, and use more energy than a right-sized unit. Match the BTUs to the space and the unit runs longer, steadier cycles that keep both temperature and humidity comfortable.
Which type fits your space
- Window units are the most cooling per dollar for a single room.
- Portable units move room to room and suit spaces where a window mount is not an option.
- Mini-split systems handle larger or multi-room areas efficiently and run quietly.
