Determine if the fan will be used continuously or intermittently. If the fan is used intermittently, it needs to be able to perform a greater number of air changers per hour (ACH).
Calculate the area that needs to be ventilated. For example, a bathroom measuring 10 feet long, 15 feet wide and 8 feet high has a total of 1,200 cubic feet of space. This example bathroom, which contains a toilet and a bathtub, will use an intermittent fan.
Divide the cubic feet of the bathroom by 60 to determine how many cubic feet per minute, or CFM, need to be ventilated. When you divide 1,200 by 60, you get 20 CFM.
Multiply the rate of CFM — 20, from the example given in Step 3 — by the number of air changes per hour (ACH). The Home Ventilating Institute recommends 8 ACH for bathrooms. Multiply 20 CFM by 8 ACH to get a total of 160. The fan needs to ventilate 160 CFM for intermittent bathroom ventilation.