Buying guide
What Size Air Conditioner Do You Need? One Formula, No Guessing
Air conditioner sizing is one multiplication and three adjustments — 20 BTU per square foot, tuned for sun, shade, and kitchens. Here's the formula, the lookup table with our pick in each size class, and the two expensive mistakes the formula prevents.
Take the quiz →The formula
Square feet × 20 = BTU, then adjust for conditions:
- Direct sun most of the day: add 10%
- Well-shaded room: subtract 10%
- Kitchen: add ~4,000 BTU (the stove and fridge are heat sources)
- Regularly more than two people in the room: add ~600 BTU per extra person
- High ceilings (over ~8 ft): size up a step — you're cooling volume, not floor
Measure the room (length × width), run the math, and buy the size class that covers the result. That's the whole trick; everything below is the lookup table and the mistakes to avoid.
The sizing table
| Room size | Room type | BTU class | Our pick |
|---|---|---|---|
| Up to 150 sq ft | Small bedroom, office, dorm | 5,000–6,000 | Frigidaire 5K — $169 |
| 150–350 sq ft | Most bedrooms, medium rooms | 8,000 | Midea U+ 8K · Windmill · GE ClearView |
| 350–550 sq ft | Living rooms, primary suites | 10,000–12,000 | Midea U+ 12K |
| 400–600 sq ft, no usable window | Open plans, sunrooms | 12,000 SACC portable | Midea Duo · Whynter NEX |
Undersizing is obvious; oversizing is sneaky
Everyone understands the undersized failure: the unit runs at 100% all afternoon and the room never quite gets there. The oversized failure is quieter and worse. A too-big AC blasts the air cold in minutes and shuts off — before it has run long enough to pull moisture out of the air. The result is a cold, clammy room that feels like a basement, plus the wear of constant start-stop cycling. Bigger is not a safety margin. Buy the size the math says.
Inverter units soften both failure modes — they ramp output up and down continuously — which is one more reason the top of our rankings is all inverters. But an inverter can't fix a genuine size-class mismatch.
Portables: do the math in SACC
All of the table above assumes window units, where BTU ratings are honest. Portable ACs advertise the inflated ASHRAE number; the real figure is SACC, typically 25–40% lower. Run the same square-footage formula, then compare it against the portable's SACC rating — a "14,000 BTU" portable is a 10,000–12,000 SACC machine at best. The portable guide covers which two are worth buying at all.
Between sizes?
At 300–350 sq ft the 8K/12K line gets blurry. Sun, a kitchen, high ceilings, or an open doorway to another space push you up to 12K; a shaded, well-insulated bedroom stays at 8K — the smaller unit is quieter and cheaper to run. The 8K vs 12K comparison walks that exact call. Or answer six questions and the quiz sizes it for you — room size is a hard gate there, on purpose.
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Frequently asked
How do I calculate what size air conditioner I need?
Multiply square feet by 20 BTU, then add ~10% for sunny rooms, subtract ~10% for shade, and add about 4,000 BTU for kitchens. A 12×15 bedroom (180 sq ft) needs roughly 4,000–6,000 BTU with adjustments; most bedrooms land in the 6,000–8,000 class.
How many square feet does a 12,000 BTU AC cool?
About 400–550 sq ft in the window-unit class — a large living room or open-plan space. On portables, 12,000 is often the inflated ASHRAE figure; check the SACC number, which is what actually predicts coverage.
Is it better to oversize an air conditioner?
No — an oversized unit cools the air before it has time to dehumidify it, then shuts off, leaving the room cold but clammy. It also wears the compressor with constant cycling. Buy the size the square-footage math indicates.
What size room does a 5,000 BTU air conditioner cool?
Roughly 100–250 sq ft depending on conditions — a small bedroom, office, or dorm. Consumer Reports rates the popular 5,000 BTU class for about 100–250 sq ft; past that, step up to an 8,000 BTU unit.