The CFM rating of a bathroom fan is a measurement of how many cubic feet of air it can ventilate out every minute. To vent a bathroom properly, the bathroom fan must have a CFM rating that is able to change the air eight times every hour.
According to the Home Ventilating Institute, a bathroom fan's CRM rating should match up to the square footage of a bathroom. Any bathroom measuring less than 50 square feet requires at least a 50-CFM bathroom fan. Larger bathrooms require more CFM so that the air inside does not become filled with excess moisture, odors and humidity.