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Home & Garden · 6 min read · Last updated September 2026
Worked example: 150 sq ft bedroom, hot climate, decent insulation: 150 × 80 = 12,000 BTU → 1.0 ton. The same room in a temperate climate with good insulation might need only 150 × 20 = 3,000 BTU — climate and insulation swing the answer 4×, which is why "one rule" sizing misleads.
| Factor | Adjustment |
|---|---|
| Sunny room | +10% |
| Shaded room | −10% |
| Kitchen | +4,000 BTU (appliance heat) |
| Each regular occupant beyond 2 | +600 BTU |
| High ceiling / open plan | Size by volume, not floor area |
An oversized unit drops temperature fast, satisfies the thermostat in minutes, and shuts off — before it has dehumidified the air. Result: cold-but-clammy rooms, more on/off cycling (wear + higher peaks), and worse comfort than the "smaller" correct unit. The AC/BTU Size Calculator applies all adjustments; the Electricity Bill Calculator estimates what your chosen tonnage costs to run.
Start at 20 BTU/sq ft for temperate, well-insulated rooms; hot climates with average insulation run 60–80 BTU/sq ft. Adjust ±10% for sun exposure and add for kitchens and extra occupants.
About 12,000 BTU (1 ton) in a hot climate: 150 × 80 = 12,000. Cooler climates with good insulation may need far less — run the full formula with your conditions.
No. Oversized units short-cycle: they cool quickly, shut off before dehumidifying, and deliver cold-but-clammy air with more wear. Correct sizing beats bigger.
Divide BTU by 12,000. 12,000 BTU = 1 ton, 18,000 = 1.5 tons, 24,000 = 2 tons.