BTU needed to heat or cool any room based on size and factors.
The rule of thumb is 20 BTU per square foot for cooling, adjusted for ceiling height, insulation, sun exposure, and occupants. One ton of cooling = 12,000 BTU/hr. A 1,500 sq ft home typically needs 2–2.5 tons of central AC.
Oversizing an AC unit causes "short cycling" — the unit cools quickly but doesn't run long enough to dehumidify properly, leaving rooms feeling clammy. A Manual J load calculation by a licensed HVAC engineer is the most accurate sizing method.