Good question yesterday–what size bath fan do you need. For bathrooms up to 100 square feet (e.g., a 10′ x 10′ room, with 8′ ceilings), the Home Ventilating Institute recommends a fan be sized to deliver 1 CFM (cubic foot per minute) of exhaust for every square foot of floor area.
For example, an 8′ x 6′ bathroom, at 48 square feet, should have a 48 CFM fan. In this case you’d probably round up to a common 50 CFM fan.
Note, this is for bathroom spot ventilation only. If you’re using you bathroom exhaust as part of a whole-house ventilation strategy, sizing changes based on the size of the home, other fans being used, and how you’ll be using this. A good home performance contractor can help you determine the right size for you.