Three ways to size a bathroom exhaust fan — area, volume, and fixture methods over the code floor

What this diagram shows

A bathroom room section on the left labeled with length, height, and depth, and an exhaust fan on the ceiling. On the right, three method cards: the area method (floor area times one CFM per square foot), the 8-air-changes volume method (length times width times height times eight, divided by sixty) for tall ceilings, and the fixture method (fifty CFM each for toilet, shower, and tub) for baths over one hundred square feet. An amber banner below marks the flat code floor of fifty CFM intermittent or twenty continuous, which the room number can never go under.