BTU / AC Sizing Calculator
Too small and the AC runs nonstop without cooling. Too big and it cycles fast without dehumidifying. This gets you in the right zone.
Sizing
BTU/hour you need
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Tons of cooling
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Recommended unit
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Why sizing matters more than people think
An undersized AC runs continuously, never cooling the room, racking up the electric bill. Annoying, but not damaging.
An oversized AC is worse. It cools the air fast and shuts off — but doesn't run long enough to dehumidify. The room feels clammy at 72°F. Frequent short cycles also wear the compressor and waste energy. "Bigger is better" is wrong for AC sizing.
The baseline
20 BTU/sqft for a "typical" room: 8-foot ceiling, average insulation, two people, moderate climate, normal sun. Then adjust:
- Ceiling height — 10% per foot above/below 8'.
- Sun — sunny rooms +10%, shaded rooms −10%.
- Climate — hot/humid +20%, cold −10%.
- Occupants — each beyond 2, +600 BTU. (A body radiates ~400 BTU at rest, more when active.)
- Kitchen — +4,000 BTU for cooking heat.
BTU → tonnage
1 ton of AC = 12,000 BTU/hour. A "3-ton system" is 36,000 BTU. Most homes pick a system size by tonnage; window/portable units are sold by BTU.
Window unit cheat sheet
- 5,000 BTU — bedroom up to ~150 sqft.
- 8,000 BTU — bedroom/living up to ~300 sqft.
- 12,000 BTU (1 ton) — large room up to ~550 sqft.
- 18,000 BTU (1.5 ton) — open-plan up to ~900 sqft.