Fans are rated by how many cubic feet per minute the fan is capable of cycling. Making sure that you have a sufficient number of air changes per hour is particularly important for bathrooms because if the humid air is not cycled out in a timely fashion, the humidity can cause wood to warp and paint to crack. You can determine the number of air changes per hour if you know the capacity of the fan and the volume of the room.
Measure the length, width and height of your room.
Multiply the length times the width times the height to calculate the volume of the room. For example, if your room is 10 feet wide, 15 feet long and eight feet high, the volume would be 1,200 cubic feet.
Check your owner's manual to determine the capacity of the fan in cubic feet per minute.
Multiply the capacity of your fan in cubic feet per minute by 60 to convert it to cubic feet per hour. For example, if your fan were capable of 200 cubic feet per minute, you would multiply it by 60 to get 12,000 cubic feet per hour.
Divide the result from Step 4 by the volume of the room from Step 2 to calculate the number of air changes per hour. Finishing the example, divide 12,000 by 1,200 to find that you would get 10 air changes per hour.