Print this calculator’s factors before you pick a head.
Oversizing feels safe. It isn't. A head that's too big short-cycles, leaves the room clammy, and dies early.
This calculator’s baseline is 25 BTU/hr per square foot. Then four factors: ceiling height, sun, insulation, and windows.
The steps. Sq ft × 25. × (ceiling ÷ 8). × sun factor. × insulation factor. + 600 BTU per window. + 10%, round up to the nearest 500. Then pick the next head in the calculator’s list.
The four knobs
Factor
Adjustment
Ceiling height
multiply by height ÷ 8 (10 ft ceiling = +25%)
Sun exposure
very sunny ×1.15 · average ×1.0 · shaded ×0.90
Insulation
poor ×1.25 · average ×1.0 · good ×0.85
Windows
+600 BTU/hr each
A head far above the load is capacity you can't use.
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