When installing air conditioning, there are two important numbers you must focus on. The first is BTU, or British Thermal Unit. This is a measurement of how cold the air conditioner can make a room. The second is CFM, or Cubic Feet per Minute. This is a measurement of how much air an air conditioner can produce per minute. To obtain the CFM requirements per room, first determine the BTU requirements for the room.
Measure the width and length of the room with a tape measure and multiply the two numbers to obtain the square footage of the room. Multiply the square footage of the room by 25 to obtain the basic BTU requirements for the room.
Add 1,000 BTU for each window in the room and 400 BTU for the average inhabitant in the room to obtain the adjusted BTU requirements for the room.
Divide the total BTU requirements for the room by 30 to obtain the necessary CFM requirements. You need 1 CFM for every 30 BTU in a room.