The right size air conditioner for your room — and why the biggest one on the shelf is usually the wrong answer.
An oversized AC cools the air so fast it shuts off before it has time to pull moisture out. You get a room that's 68° but clammy, plus the wear of constant on-off cycling. A correctly sized unit runs longer, quieter, and leaves the room actually comfortable. Resist the "one size up, just in case" instinct.
The math here follows the standard Energy Star rules of thumb: 20 BTU per square foot, scaled for ceilings taller than 8 ft, minus 10% for shady rooms, plus 10% for sunny ones, +600 BTU per person beyond two, and +4,000 BTU for kitchens. We then round up to the nearest common unit size (5,000 / 6,000 / 8,000 / 10,000 / 12,000 / 14,000 / 18,000 / 24,000), since you can't buy a 9,730 BTU unit.
Caveats worth knowing: bad insulation, big west-facing windows, or open floor plans can push the real need higher — and if your number lands above 24,000 BTU, consider two units or a mini-split sized by a pro rather than one giant window shaker.
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Cooling capacity is measured in BTUs, and the rule of thumb is about 20 BTU per square foot of room. But square footage is only the start: a sunny room, a kitchen, or a space that's regularly crowded needs more, while a shaded room needs less. Getting the size right matters more than people think — and with air conditioners, bigger is not better.
A 300-square-foot living room needs roughly 6,000 BTU. Add about 10% if it's very sunny, another 600 BTU per regular occupant beyond two, and around 4,000 BTU if it's a kitchen. An oversized unit cools the air fast but shuts off before it removes humidity, then cycles on and off — leaving the room cold and clammy while wearing out the compressor. A right-sized unit runs longer, steadier, and drier.
Why is oversizing bad?
Short cycling means poor humidity control, uneven temperatures, more wear, and wasted energy. Matching capacity to the room is the goal.
What raises the BTU I need?
Strong sun, lots of windows, high ceilings, a kitchen's heat, and more people all push the number up.
Does ceiling height matter?
Yes — the rule assumes standard 8-ft ceilings. Tall ceilings mean more air to cool, so size up.