A standard 5’ x 8’ bathroom requires a fan of between 50 and 70 CFMs, while a larger bathroom requires more than 100 CFMs. Typically, you need a minimum of 1 CFM per square foot for bathrooms up to 100 sq.ft. Over that, add 50 for each water appliance. Every bathroom should have at least 10 to 20 CFMs more if you vent with ducts.