Read this part first
This is a conversation tool, not a sizing tool. It gives you a rough cooling capacity figure from square footage, climate zone, and building condition so you can respond intelligently on a first phone call or sanity-check a system that is already installed.
Equipment selection requires a Manual J load calculation. That is not a formality. It is the difference between a system that works and a callback you cannot fix with a bigger unit.
Why oversizing is the expensive mistake
An oversized system satisfies the thermostat fast and shuts off. Short cycles mean the coil never runs long enough to pull humidity out of the air, so the house reads cold and clammy and the customer is unhappy in a way that no amount of airflow balancing will resolve.
The compressor pays for it too. Repeated starts are the hardest thing a compressor does, and a chronically short-cycling unit fails years early. Undersizing is also bad, but it fails visibly and honestly. Oversizing fails quietly while the customer blames you.
What the rule of thumb misses
Square footage is one variable among many. Real load depends on orientation, window area and glazing type, ceiling height, infiltration, insulation levels, duct condition and location, and internal gains from people and equipment.
Two identical floor plans on the same street can differ by half a ton based on which way they face and what the previous owner did to the attic. Use this to start the conversation, then do the calculation.