Match your room’s square footage to the ENERGY STAR chart, then adjust for sun, extra people, and kitchen heat.
How to size air conditioner for a room comes down to one measurement: the room’s square footage, matched to the right cooling capacity in BTUs per hour. That’s the starting point — sunlight, occupants, and kitchen heat all change the number, and ENERGY STAR publishes the exact adjustments to make.
Start With Square Footage
Measure the room’s length and width in feet and multiply the two numbers. That gives you the square footage that drives the whole sizing decision.
Room Size vs. BTU Requirements
These figures assume average sun exposure, up to two occupants, and no special heat sources.
| Room Size (Sq Ft) | Recommended Capacity (BTU/h) |
|---|---|
| 100–150 | 5,000 |
| 150–250 | 6,000 |
| 250–300 | 7,000 |
| 300–350 | 8,000 |
| 350–400 | 9,000 |
| 400–450 | 10,000 |
| 450–550 | 12,000 |
| 550–700 | 14,000 |
| 700–1,000 | 18,000 |
| 1,000–1,200 | 21,000 |
| 1,200–1,400 | 23,000 |
| 1,400–1,500 | 24,000 |
| 1,500–2,000 | 30,000 |
| 2,000–2,500 | 34,000 |
If your room falls between two ranges, size up to the larger capacity.
Adjust For Sun, Occupants, And Kitchens
Square footage alone is only the first pass. ENERGY STAR’s guidance includes three adjustments that matter more than most buyers realize:
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase capacity by 10%.
- Each additional person beyond two: add 600 BTU/h.
- Kitchen installation: add 4,000 BTU/h.
If the room has big south- or west-facing windows, treat it as sunny even when the overall house is shaded.
Oversizing: The Mistake That Wastes Money
Most people buy too much air conditioner, not too little. An oversized unit cools the air fast, then shuts off before it has run long enough to remove humidity — so the room feels cold, damp, and clammy, and the compressor cycles on and off all day, eating extra energy. In smaller rooms especially, the chart is there to right-size you, not to push you upward.
One caution on portable units: they’re often labeled with DOE/SACC BTU ratings instead of the same window-AC chart numbers, so don’t compare marketing BTU figures across portable and window units without checking which rating standard is being used. And if you’re narrowing down actual models, our tested roundup of the best one-room air conditioners covers the units that handled real-world cooling best.
FAQs
Is It Better To Oversize Or Undersize A Room AC?
Neither. A slightly undersized unit runs longer but dehumidifies better, while a badly oversized unit short-cycles and leaves the room clammy. Match the square footage on the ENERGY STAR chart, apply the sun and occupant adjustments, and choose the next size up only when your room sits between two ranges.
Does Ceiling Height Change The BTU Number?
Yes. The ENERGY STAR chart assumes standard 8-foot ceilings. If your room has cathedral or tall ceilings, the cubic volume is larger than the square footage suggests, so step up at least one capacity range — and more if the room gets strong sun through tall windows.
Will A Bigger AC Cool A Room Faster?
It cools the air faster, but that’s not the same as cooling better. The compressor shuts off before the room dehumidifies, so the space feels chilly and sticky at the same time. Slower cooling from a right-sized unit leaves the room drier and more comfortable, and it also uses less energy over a full summer.