Use the area of a room to determine the proper air conditioner size estimate, which is measured in British thermal units per hour. A 100- to 150-square-foot room needs a 5,000 BTU per hour unit, while a 400- to 450-square-foot room needs a 10,000 BTU per hour unit.
The range for BTUs sometimes needs adjustments based on specific conditions in the room. The BTU calculation decreases by 10 percent for a heavily shaded room and increases by 10 percent for a sunny room, according to recommendations from Energy Star. For rooms with more than two occupants, the BTU calculation increases by 600 for every extra person. A kitchen air conditioner needs an extra 4,000 BTUs to compensate for the heat and humidity in the room.
Sizing an air conditioner depends on more than just the cooling capacity of the appliance because the unit also removes humidity from the room. If the air conditioner is too large for the space, it doesn't remove enough humidity, so the room feels cool and damp. A unit that is too small for the space can't keep up with the cooling demands. Product specifications for each air conditioning unit list the estimated coverage area to help consumers select the correct size.