A bathroom without a ventilation fan is like a fireplace without a chimney: If you fail to pull the moisture generated in the bathroom out of there, it will migrate into the walls and grow mold and mildew, or blister paint and peel wallpaper.
Bath vent fans are rated by how many cubic feet of air they can move in one minute, known as the CFM rating. To determine which size fan to buy for your job, multiply the room's square footage by 1.1. For example, a 100-square-foot bath would require a 110 CFM-rated fan. Fans also have a sound rating, measured in sones. (A modern refrigerator operates at about one sone.) Vent fans range from as low as 0.5 sone up to about 6.0 sones. You’ll find both the CFM and sone ratings printed on the vent fan’s box.