Measure the length, width and height of each room you want to cool and record the measurements.
Multiply the length, width and ceiling height measurements of a room to get the cubic feet of the room. For example, a room measuring 12 by 12 feet with 10-foot ceilings would have a cubic foot measurement of 1,440.
Add the cubic foot measurements of all the rooms together to get a total cubic foot measurement of the area you want to cool.
Divide the total cubic foot measurement by two. The result is the amount of Cubic Feet per Minute (CFMs) that you want your evaporative cooler to be rated to cool the desired area effectively. For example, if your total cubic feet measurement was 15,000, you want an evaporative cooler rated at 7,500 CFMs or higher.