How AC BTU sizing works
Room air conditioners are sized by the floor area they cool. The ENERGY STAR chart starts at 5,000 BTU for 100–150 sq ft and rises to 34,000 BTU for 2,000–2,500 sq ft — roughly 20 BTU per square foot for typical rooms. Then adjust: reduce capacity by 10% for a heavily shaded room, increase it by 10% for a very sunny room, add 600 BTU for each person beyond two who regularly uses the room, and add 4,000 BTU if the unit cools a kitchen.
Worked example
A 12 × 16 ft bedroom is 192 sq ft, which falls in the 150–250 sq ft row: 6,000 BTU. A 400 sq ft living room is 9,000 BTU; if it is very sunny (+900 BTU) and four people use it regularly (+1,200 BTU), the estimate is 11,100 BTU, so a 12,000 BTU unit is the practical choice.
Why bigger is not better
An oversized air conditioner cools the air quickly and shuts off before it has removed enough moisture, leaving the room cold and clammy, and it cycles on and off more often. An undersized unit runs constantly and cannot reach the set temperature on hot days. Choosing the capacity closest to the estimate gives the best comfort and efficiency.
Central air and whole houses
This chart is meant for single rooms and window, portable or through-the-wall units. For central air or ductless systems covering a whole house, HVAC contractors use a Manual J load calculation, which also accounts for insulation, windows, climate and air leakage.