In order to ensure healthy indoor air quality, it's essential to have properly sized ventilation, especially in airtight homes. Additionally, as the modern trend to have larger bathrooms remains popular, the requirement for proper air ventilation is even more important. That said, while most people know that's it's important to have (and use) a bathroom fan, not as many people understand how to accurately size a fan for their bathroom space.
There are several different ways to compute the proper size of a bathroom fan needed to properly vent a bathroom. Let's take a look at three of the most common ways.
Method 1: 8 Complete Air Changes Per Hour Method
This method assumes that the objective of any bathroom ventilation goal is 8 complete air changes per hour.
Calculation: Square footage x ceiling height = total cubic foot ventilated
Total cubic foot to be ventilated divided by 60 minutes (1 hour)
Result x 8 complete air changes per hour = CFM air movement to ventilate bathroom.
Example: 11 feet wide by 14 feet long bathroom with 8 foot ceilings.
11 ft X 14 ft X 8 ft = 1,232 cubic feet
1,232 cubic feet divided by 60 minutes = 20.53
20.53 x 8 complete air changes per hour = 164 CFM needed
Method 2: Bathrooms Less Than 100 Square Feet (requires one CFM per square foot of bathroom - minimum 50 CFM)
Calculation: length x width = CFM needed
Example: a 8 foot by 9 foot bathroom will need 72 CFM
Method 3: Bathrooms Greater Than 100 Square Feet (factors in number of fixtures in bathroom, such as showers, tubs, sinks, toilets)
Requirements: Allow 50 CFM for each standard tub, shower, or toilet and 100 CFM for jetted and whirlpool tubs
Example: Bathroom has one standard toilet (uses 50 CFM), one standard shower (uses 50 CFM), and one whirlpool tub (uses 100 CFM).
50 CFM + 50 CFM + 100 CFM = 200 total CFM of ventilation required
Keep in mind that if you also have a hot tub, steam room, or sauna, you'll need to increase the size of the bathroom ventilation fan to compensation for additional moisture.