Size an air conditioner or heater to a room — from its square footage, with adjustments for sunlight, people, and kitchens.
Uses the DOE rule of thumb (~20 BTU per sq ft for cooling; 30–40 for heating). Real sizing depends on insulation, climate zone, windows, and air sealing — a Manual J load calculation is the professional standard. Estimate only.
A rough industry rule is about 20 BTU of cooling per square foot, adjusted up for sun, extra people, tall ceilings, and kitchens. Oversizing wastes money and short-cycles (cools without dehumidifying); undersizing never keeps up. This gets you to the right ballpark before a load calculation.
Cooling BTU ≈ area × 20, adjusted for sun, people, ceiling, and kitchen
12,000 BTU/hour = 1 ton. A 1.5-ton AC is 18,000 BTU. The tons figure helps when shopping central and mini-split systems.
An oversized AC cools fast but doesn't run long enough to remove humidity, leaving rooms cold and clammy — and it costs more. Right-sizing is more comfortable and efficient.
Related: square footage calculator · electricity cost calculator · all free calculators.
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