Measure the length of the room. If you are measuring this alone and are having difficulty holding the tape measure in place, break the measurement into two parts, and add them together.
Measure the width of the room. Again, break into two measurements if you're having difficulty.
Measure the height from floor to ceiling. Round up to the nearest foot for easier calculation.
Multiple length times width times height of the wall to determine the cubic feet of the room.
Divide the cubic feet by 60, the number of minutes in an hour. Your goal is to achieve 8 air changes per hour, which is why you divide the cubic feet by 60 minutes.
Multiply by eight since eight air changes per hour is your goal.
Find a fan at your local hardware store that is close to this number; round up to the higher number if possible.