The capacity sizing rule of thumb is a quick way to estimate how much cooling or heating a space needs, based only on its floor area. The most common version is about 20 BTU (British Thermal Units) of cooling per hour for each square foot of living space. The U.S. Department of Energy uses this figure as a starting point for room air conditioners.
For a buyer, the rule is a sanity check, not a final answer. It gives you a rough target so you can spot a quote that is wildly oversized before you spend $5,000 or more. At 20 BTU per square foot, a 1,500-square-foot home works out to about 30,000 BTU, or roughly 2.5 tons (one ton equals 12,000 BTU per hour). That math also explains the related shortcut of one ton per 500 to 600 square feet.
The catch is that floor area ignores almost everything that actually drives the load: insulation, the number and direction of windows, ceiling height, how many people live there, and the local climate. A shaded, well-insulated house needs far less than a sun-baked one of the same size. This is why the rule of thumb only points you to the ballpark.
The accurate method is a Manual J load calculation — the industry-standard procedure from the Air Conditioning Contractors of America that prices in all of those factors. A careful contractor runs Manual J and can show you the printout. ENERGY STAR's own adjustments hint at why detail matters: add 600 BTU for each person beyond two, add 4,000 BTU for a kitchen, and adjust up to 10 percent for a very sunny or heavily shaded room.