Cooling load scales with the volume of air being conditioned and the wall area leaking heat in — both grow with height. A 12×15 room at 8 ft holds 1,440 cu ft; at 11 ft, 1,980 cu ft — 37% more air. The 10%-per-foot rule approximates this well for normal rooms; our calculator caps the multiplier at 1.5× for very tall spaces, where stratification (hot air pooling overhead) changes the physics.
For vaulted ceilings, use the average height. Double-height halls and stairwell-open living rooms behave differently: destratification (ceiling fans on low, reversed) often recovers comfort more cheaply than tonnage. Indian rooms at the common 9.5–10 ft get this adjustment automatically in India mode.
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Yes — roughly 10% more capacity per foot above 8 ft. A 10 ft room needs ~20% more BTU than the same floor area at 8 ft.
Yes, twice over: it shrinks the cooled volume and insulates against roof heat — often a half-ton saving in top-floor Indian rooms.
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