There is a lot of math involved to calculate the capacity of an exhaust fan. However, if you measure everything accurately the math shall bring a full and simple answer.
The first thing is to measure the dimensions of the room with the exhaust fan—length, width, and height. Next, calculate the room's volume in cubic feet by multiplying height by width by length, and convert accordingly.
Personalize the Math
Now, determine how many times per hour you would like to hear the air being cycled out of the bathroom—the most common number is eight. Next, divide sixty by that number, be it eight or whatever have you. This will establish how many minutes it should take your fan to cycle out the air. Afterwards, divide the volume of the bathroom by the minutes you determined just now. This new number is the minimum rating for your bathroom exhaust fan. Assuming the answer is about eighty, it suggests that the fan has a capacity of a minimum of eighty cubic feet per minute.