When choosing a bathroom exhaust fans, as shown in Figure 1, it is important that the fan chosen be of the correct size in its ability to exhaust sufficient quantities of air (CFM) from the bathroom.
Figure 1 - Bathroom Exhaust Fan
Bathroom exhaust fans are designed to remove excess humidity and odors from the room. In order for the bathroom exhaust fan to work properly it must exhaust an amount of air in a period of time. The calculation is CFM (cubic feet per minute). A fan with a rating of 100 CFM, will exhaust 100 cubic feet of air per minute.
The exhaust fan's rating in CFM is based on laboratory calculations that assume that their is no duct work system connected to the exhaust fan, a situation that is not based on reality as it is impossible to install a bathroom exhaust fan properly without having some duct work and a roof, wall or eave termination cap.
Figure 2 - Exhaust fan roof cap
Figure 3 - Exhaust fan wall cap
Figure 4 - Exhaust fan eave cap
Note: If your bathroom has multiple ceiling heights use the highest measurement.
Multiply these numbers together to obtain the number of cubic feet. Width x Height x Length.
Divide the cubic feet by 60 (minutes).
Multiply that number by 8, which is the number of times you want to remove the air from your bathroom every hour.
As an example. If your bathroom was 8 feet high, 6 feet wide and 10 feet long.
The calculation would be 8 x 6 x 10 ÷ 60 x 8 = 64CFM
If the number is less than 50 you would still use a 50CFM fan as this is a common building code requirement.
Note: It is best to check with the building department in your jurisdiction to find out what their requirements are.
A convenient table of widths, heights and lengths has been created so that you do not have to bother with the math.
If you have a bathroom that has a larger area than 100 square feet (width x length). You can calculate the requirement based on the number and type of bathroom fixtures are in the room.
Toilet, Stall shower, Bathtub are 50CFM each
Whirlpool style bathtub is 100CFM
As an example. If you have a bathroom that is larger than 100 square feet and have a Jacuzzi, toilet, stall shower and bathtub you would require a 250 CFM bathroom fan.