BTU Calculator
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BTU Needed
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Base BTU—
Adjustments—
BTU Required
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Tons (AC)
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Room Area
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Room Volume
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Suggested Window AC
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Suggested Central AC
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BTU Sizing Rule of Thumb
Start with 20 BTU per square foot as a baseline. Adjust up for high ceilings, lots of windows, hot climates, and kitchens. Adjust down for well-insulated or north-facing rooms. Oversizing is just as problematic as undersizing — an AC that's too large short-cycles and doesn't dehumidify properly.
BTU vs. Tons
One ton of cooling = 12,000 BTU/hr. Residential central AC units range from 1.5 to 5 tons (18,000–60,000 BTU). Window units typically cover 150–550 sq ft (5,000–12,000 BTU).
Heating vs. Cooling
This calculator sizes for summer cooling. For heating, the same BTU figure applies but climate zone matters more — a Manual J calculation by an HVAC professional is recommended for heating in cold climates.