To calculate the minimum cubic feet per minute rating needed for an exhaust fan, multiply the bathroom's length, width and height, and then multiply that result by 0.13. The final answer is the minimum CFM rating for the fan.
For instance, if a bathroom is 10 feet long, 8 feet wide and 9 feet tall, the volume of the room is 720 cubic feet. When that is multiplied by 1.3, the result is 93.6. Therefore, the minimum CFM rating needed is about 94. This formula is appropriate for bathrooms with one sink and one shower head. If a room has more, the exhaust fan needs to be more powerful to deal with the increased air flow.
There is also a rule of thumb to estimate the CFM rating needed. This uses the area of the room, which is found by multiplying its length by its width. A bathroom that is 50 square feet or less needs a 50 CFM fan. With larger rooms, at least one CFM for each square foot is required. Therefore, a bathroom with an area of 70 square feet needs a fan with a minimum CFM rating of 70.
Fan ratings typically run between 50 and 110 CFM. The appropriate-size fan for a bathroom can replace the room's air eight times in one hour.