Bathroom fans work by recirculating air in your bathroom with outside air. These ventilating fans are necessary in windowless bathrooms because the accumulated humidity from showers, baths or faucets can lead to mold and mildew damage. Bathroom fans come in different sizes, and if you plan on installing one, you'll need to know whether it is adequate for a bathroom in your home. A few quick measurements and a simple calculation can help you determine the right size fan for your bathroom.
Using a tape measure, calculate the total volume of your bathroom by multiplying the length times the width times the height in feet. Be sure to include bath and shower space in your calculations. If you have access to your home's original architectural plans, you can crib the measurements from there, but a tape measure works just as easily.
Divide the volume of your bathroom in feet by 7.5. The result is the CFM, or cubic feet per minute rating required for your bathroom fan. The CFM represents the airflow required to completely exchange the air in your bathroom 8 times in one hour. So if you have a bathroom that is 12 feet long, 15 feet wide and 10 feet tall, then it has a volume of 1800 cubic feet (12 x 15 x 10) and a CFM of 240 (1800/7.5).
Alternately, you can determine the CFM by dividing the total volume of your bathroom by 60--the number of minutes in an hour--and then multiplying the results by 8--the number of complete circulations required for a bathroom fan to function effectively. So if your bathroom is 20 feet long, 8 feet wide, and 9 feet tall, it has a volume of 1440 cubic feet (20 x 8 x 9). Dividing that number by 60 equals 24 (the number of cubic feet which must be recirculated each minute for one complete recirculation in an hour) and multiplying that number by 8 (the number of times the air must be recirculated per hour to keep the bathroom mold free) equals 192: the CFM required by any fan installed in that bathroom.
Check the CFM rating on the packaging of any bathroom fan you intend to purchase. It needs to match the CFM value you calculated in the previous step.
If you don't have time to make any formal calculations, you can determine the rough CFM for your bathroom fan based on the number of fixtures the bathroom contains. Each toilet, shower and bath tub needs about 50 CFM each, while a jet tub needs 100 CFM. While it won't be as effective as a fan with a proper CFM, you can install a smaller fan directly above these fixtures (for instance, a 50 CFM fan above the toilet) in order to satisfy a minimum need for ventilation.