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Buying a room air conditioner that is too big or too small is a common mistake. ENERGY STAR, the US government efficiency program, gives a simple method: measure the floor area, then match a capacity in BTU per hour.
This guide is based on the public sources listed at the end. We have not tested the products or services mentioned.
This is a short reference page. The full guide is Wi-Fi, Cooling and Smart Home Basics.
Step 1: measure the room
Multiply length by width in feet. (One square meter is about 10.76 square feet.) The table below assumes 8-foot ceilings.
| Floor area (sq ft) | Capacity (BTU/h) |
|---|---|
| 100 to 150 | 5,000 |
| 150 to 250 | 6,000 |
| 250 to 300 | 7,000 |
| 300 to 350 | 8,000 |
| 350 to 400 | 9,000 |
| 400 to 450 | 10,000 |
| 450 to 550 | 12,000 |
| 550 to 700 | 14,000 |
| 700 to 1,000 | 18,000 |
Larger rooms continue up the same table, to 34,000 BTU/h for 2,000 to 2,500 sq ft.
Step 2: adjust
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase by 10%.
- More than two regular occupants: add 600 BTU for each extra person.
- Kitchen: add 4,000 BTU.
- Taller ceilings may need more.
Why bigger is not better
ENERGY STAR says a unit that is too big costs more, wastes energy, cools poorly and leaves the room damp, because it cycles off before it removes humidity.
Efficiency and installation
- Look for the ENERGY STAR label. Variable-speed compressors adjust output instead of switching on and off.
- The efficiency figure on listings is CEER: cooling output divided by power input, standby included; higher is better. Thresholds change by version and size class, so check the product finder before comparing numbers.
- Seal around the unit with the included insulation, level it for drainage and keep lamps and TVs away from the thermostat.
What about a smart controller?
A smart controller does not change the size you need. See our guide to choosing a smart AC controller.
Last checked: 15 October 2026. Sources: ENERGY STAR, room air conditioners.
