Rough BTU/hr sizing for a room air conditioner or heater.
A rule-of-thumb estimate (20 BTU/sq ft base) — get a professional load calculation for a real installation.
A taller ceiling means more air volume to heat or cool for the same floor area. The calculator applies a multiplier — 1.0 at 8 ft, 1.125 at 9 ft, 1.25 at 10 ft — on top of the 20 BTU/sq ft base rate.
People add heat load to a room. Beyond the first 2 occupants, the calculator adds 600 BTU/hr for each additional person, since more bodies in the room means more cooling capacity is needed to keep it comfortable.
It's a rough rule-of-thumb, not a substitute for a Manual J load calculation. Real BTU needs also depend on insulation, window count and orientation, climate zone, and ceiling fans — get a professional load calculation before buying equipment for a real installation.