The short answer
A bathroom fan should exhaust outdoors and be sized to the room. The common guideline for bathrooms up to 100 ft² (9 m²) is 1 CFM (cubic feet per minute of airflow) per square foot, with a minimum of 50 CFM, so an 85 ft² (8 m²) bath needs at least 85 CFM.
What we look for
- Ducted to the outdoors, not into an attic or wall cavity
- Sound rated at 1.0 sone or lower (a sone is a loudness rating; lower is quieter), so people actually run it
- An automatic humidity sensor or timer to keep it running 20 minutes after a shower
- ENERGY STAR certification for lower running cost
Why it matters
Lingering moisture feeds mold on grout, ceilings and window frames. If you smell mustiness, the fan is likely undersized, blocked or switched off too soon.
The size is only useful if the duct can deliver it
Long, narrow or sharply bent ductwork increases resistance and reduces real airflow. Use the duct diameter the fan maker specifies, keep the route short and smooth where possible, and make sure the exterior damper opens freely. A fan labeled 85 CFM cannot provide that flow through a restricted or disconnected duct.
Run time and sound matter
A quiet fan is more likely to stay on long enough to dry the room. Look for 1.0 sone or less when the bath is near a bedroom, and use a timer or humidity control so the fan continues after the shower. Twenty minutes is a useful starting point, but the correct run time is the time needed for humidity and wet surfaces to return toward their earlier condition.
Interactive tool
Bathroom fan CFM calculator
Minimum rated airflow
85 CFM
For bathrooms up to 100 ft², using 1 CFM per square foot with a 50 CFM minimum. Larger baths need fixture-based sizing.
Questions, answered
How many CFM should a bathroom fan have?
For bathrooms up to 100 ft², the Home Ventilating Institute recommends roughly 1 CFM per square foot, with a 50 CFM minimum. Larger bathrooms are sized by adding the recommended airflow for each fixture.
How long should a bathroom fan run after a shower?
About 20 minutes is a useful starting point. Longer may be needed when surfaces remain wet or humidity stays elevated. A timer or humidity control makes consistent run time easier.
Can a bathroom fan vent into the attic?
No. It should discharge outdoors. Venting moist air into an attic or wall cavity can move the moisture problem into a concealed part of the building.
Sources
- Home Ventilating Institute: Bathroom ventilation guidance
- ENERGY STAR: Product Finder
- US EPA: A Brief Guide to Mold, Moisture and Your Home
General information, not medical advice. Sources and specifications were reviewed on October 1, 2026. We do not yet conduct hands-on testing.