Room air conditioners and heaters are sold by capacity in BTU per hour. Too small and the room never gets comfortable. Too big and an AC short-cycles, leaving the room cold and clammy because it never runs long enough to remove humidity.
Cooling rule of thumb
Start at about 20 BTU per square foot of floor area. Add 10% for a very sunny room and take 10% off for a shaded one. Add 600 BTU for each regular occupant beyond two, and about 4,000 BTU for a kitchen. 12,000 BTU/h is one ton.
Heating rule of thumb
Heating needs depend heavily on climate: from around 30 BTU per sq ft in hot zones to 60 or more in very cold ones, adjusted for insulation.
Whole-house systems
For central systems and heat pumps, contractors should do an ACCA Manual J load calculation. Rules of thumb routinely oversize whole-house equipment.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.