How to Size a Bathroom Exhaust Fan

In order to keep bathrooms free from mold and mildew, bathroom exhaust fans remove moisture and odor quickly and easily. The real secret is getting the right sized exhaust fan for the proper room size.

Bathrooms are especially susceptible to moisture and odors. Because of these conditions, mold and mildew are an ever-possible threat and must be removed to keep a fresh and odor-free room in your home. Because of the need for removal of these elements, a ceiling-mounted exhaust fan should be installed. These exhaust fans are rather heavy and are mounted directly to the ceiling joist via mounting tabs on the exhaust fan. Ceiling exhaust fans often come in models with just a fan or a fan/light combination. Either model is powered from a 120-volt circuit that is fed from a switch near the bathroom door. The bathroom exhaust fan is vented via a 4” vent opening which is connected to a pipe or flex hose leading outside the house where it is discarded.
Now that we all know the theory behind using a bathroom exhaust fan, which one is right for my house? Are they all the same? Obviously, the answer is no. Bathroom exhaust fans are rated by the amount of air it can remove per cubic feet per minute (cfm). This equation should be used for bathrooms which are 100 square feet or less. To determine the square footage of a room, multiply the length times the width of the room. For example, a common bathroom dimension may be 5 X 8 or 40 square feet. This bathroom would require an exhaust fan of 40 cfm or higher.

In the case of larger bathrooms, over 100 square feet, exhaust fans are rated by the fixtures in the bathroom and the amount of them. To calculate this formula, you’ll need to add the required cfm ratings per fixture. They are rated by this figures: a bathtub requires 50 cfm, a jetted tub, like a whirlpool tub, has to have 100 cfm, a shower needs 50 cfm, and a toilet should have a removal value of 50 cfm. Therefore, having a shower and a toilet in a bathroom would require 100 cfm-rated exhaust fan, but a whirlpool tub, toilet, and shower would require a fan with a cfm of 200 or greater to remove the moisture and odor from the space.

If your home happens to have an enclosed toilet or shower, you will want to place a separate exhaust fan for that small area. Remember to keep switches out of reach of the side of tubs and showers for electrical safety. The idea is to not be standing in contact with water while switching on a switch for a light or whirlpool timer that could ultimately cause an electrical shock, or worse yet, electrocution.

So before you rush out and buy the cheapest bathroom exhaust fan on the market, consider the area of your bathroom and the reason that you are buying the fan to begin with. If the fan isn’t large enough, it won’t be able to do the job. This will lead to excessive moisture in the bathroom and future problems. A well thought out plan will provide adequate ventilation and the right sized bathroom exhaust fan.