The bedroom almost always wants a 9k head. Not a 18k because it was on sale.

Oversizing feels safe. It isn't. A head that's too big short-cycles, leaves the room clammy, and dies early.

Baseline is 20–30 BTU/hr per square foot (25 in the middle), from the same Manual-J-inspired rule of thumb on the calculator. Then four published adjustments — ceiling height, sun, insulation, windows.

Use it in 10 minutes. Sq ft × 25. × (ceiling ÷ 8). × sun factor. × insulation factor. + 600 BTU per window. + 10%, round up to the nearest 500. Then pick the next standard head size.

Typical rooms → head size

Room	Sq ft	Typical BTU/hr	Mini-split head
Bedroom	130–180	4,000–6,000	9k
Large bedroom / office	200–300	6,000–9,000	9k
Living room, average	350–500	10,000–14,000	12k
Great room / open plan	600–800	17,000–23,000	18k–24k
Whole small apartment	800–1,200	23,000–34,000	24k–36k multi-zone

Assumes 8 ft ceilings, average sun and insulation. Not a substitute for ACCA Manual J.

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

Past ~30% above load you're paying for capacity you can't use. Undersizing just runs longer on the worst days — cheaper mistake.

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