An air conditioner's capacity must be sized to fit the square footage of the room. If the air conditioner is too large, it turns off and on too quickly and wastes energy. If it is too small, it can't cool the room to a comfortable temperature.
Multiply the width by the length of the room. This is the square footage of the space.
Consult with the salesperson or look at a sizing chart on a website, such as Energy Star, to find the appropriate size of air conditioner for the room.
Adjust the size of the air conditioner according to the room's conditions. Subtract 10 percent from the air conditioner's capacity if it is placed in a shady room and add 10 percent if it is installed in a sunny room. Add 4,000 British thermal units, or BTUs, to an air conditioner used in a kitchen.