AC / BTU sizing calculator
Air conditioner size for a room.
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How it's calculated
The starting point is 20 BTU per hour for every square foot of floor, the rule the US Energy Star guidance is built on. That is scaled for ceilings taller than 8 ft, reduced 10% for a shaded room or raised 10% for a very sunny one, then 600 BTU is added for each person beyond two and 4,000 BTU for a kitchen. Divide by 12,000 to get tons, the unit used for central air and mini-splits.
Is a bigger air conditioner better?
No. An oversized unit cools the air quickly and shuts off before it has pulled the humidity out, so the room feels clammy, and the short cycles wear the compressor. Pick the size closest above the result, not the biggest you can afford.
How many BTU do I need for a 12×12 room?
About 3,000 BTU on the rule, so a 5,000 BTU window unit, the smallest sold, is plenty. A 300 sq ft bedroom needs about 6,000; a 500 sq ft living room 10,000 to 12,000.
Does insulation or climate change the answer?
Yes. A poorly insulated room, a top floor under a hot roof, or a Phoenix summer can need 10–20% more, while a well-insulated room in a mild climate needs less. The tool uses typical values; adjust the sun setting to lean one way or the other.
Results are estimates for informational purposes only and are not financial, medical or legal advice.