CFM data is provided with the exhaust fans. This data is actually cubic feet per minute of air movement. However, the rules vary depending on whether the bathrooms are smaller or larger. Speaking of small bathrooms, the basic principle is that you need 1 CFM for a square foot. In other words, if you have a bathroom measuring 6 feet x 8 feet or the equivalent of 48 square feet, you need 48 CFM at least. For larger bathrooms, that is to say, those with a larger square surface, the principle to be applied remains the same.