Determining Square Footage

• Window mount air conditioners work best when used in a specific room. Central air conditioning units provide cooler spaces all around your home because the units work through the vents in your home. Before installing an AC unit, you must determine the square footage. Multiply the width of the room by the length of the room for rooms with a flat plan, such as a square-shaped room. For rooms with unusual angles or shapes, you must determine the square footage of each space in the room and add those measurements together.
  
  

Smaller Rooms

• Air conditioning units produce British thermal units (BTUs) and the amount of BTUs produced relates to the size of the room. For a room that is smaller than 150 square feet, you need a unit that produces 5,000 BTUs every hour. The sizing is listed directly on the box or in the paperwork that comes with the unit. The number of BTUs needed rises based on the size of the room. For a room of 250 square feet, you need a unit that produces 6,000 BTUs, while a 7,000 BTU air conditioner is best for a room of 300 square feet or less.
  
  

Larger Rooms

• Larger rooms require air conditioners that produce a higher number of BTUs. For a room of 2,500 square feet, you need a machine that creates 34,000 BTUs. A machine that puts out 30,000 BTUs is suitable for a room of 2,000 square feet and 24,000 BTUs is needed for a room of 1,400 square feet. In general, add 1,000 BTUs to the sizing for every additional 100 square feet of space in the room above 200 square feet.
  
  

Considerations

• According to Energy Star, you need to increase the size of the unit based on certain factors in the room. For rooms that have multiple people or have a high amount of natural sunlight, you must buy a larger size. Energy Star recommends adding 600 BTUs to the size needed for the room for every person over two in the room. For air conditioners installed in kitchens, you need up to 4,000 more BTUs than you would ordinarily use, based on the heat coming from the appliances.