BTU, tonnage, and airflow — rule-of-thumb sizing for AC, heat, and ventilation.
The instinct with heating and cooling is to round up, and it is the wrong instinct. An oversized unit satisfies the thermostat fast, shuts off, and starts again — short-cycling that leaves humidity in the air, wears the equipment, and costs more to run than the right-sized unit would. These calculators give you the rule-of-thumb figure that tells you which size class you are in: room BTU, tonnage, furnace output, mini-split capacity, and the ventilation numbers that go with them. Treat the result as the sanity check it is. A room-by-room Manual J load calculation accounts for your insulation, windows, and orientation, and it is what settles the question.