Bathroom Exhaust Fan CFM Calculator
Get the right exhaust fan CFM for your bathroom size, using HVI sizing rules for area and fixture count.
Fixture checkboxes only change the result once the room is over 100 sq ft. Duct length and bends reduce a fan's real airflow below its rated CFM, so size up if you're between two models or running a long duct run.
How it works
This follows the Home Ventilating Institute's standard sizing rules. For a bathroom of 100 sq ft or less, the target is 1 CFM per square foot of floor area, with a 50 CFM floor even for tiny powder rooms. Above 100 sq ft, the rule switches from area to fixtures: add 50 CFM for a toilet, 50 for a shower, 50 for a tub, and 100 for a jetted tub, since a bigger room needs enough airflow at each moisture source rather than just a flat rate per square foot.
Worked example: a 40 sq ft bathroom with a toilet and shower falls under the area rule. 1 CFM per sq ft would only call for 40 CFM, but the 50 CFM floor takes over, so the fan needs to move 50 CFM. Now take a 120 sq ft primary bathroom with a toilet, shower, and a regular tub. That crosses the 100 sq ft line, so it switches to the fixture rule: 50 for the toilet, 50 for the shower, and 50 for the tub, for 150 CFM total. Swap that tub for a jetted tub instead and the total rises to 200 CFM, since a jetted tub's pump and splash add more moisture to clear.
FAQ
Can I just buy the biggest fan and call it done?
Bigger isn't automatically better. An oversized fan in a small room can be loud and waste energy, and it won't fix poor ducting. Match the CFM to the room and fixtures, then pick a model with a sone rating you can live with.
Why do fixtures only matter above 100 sq ft?
Below that size, one central rule (CFM per square foot) already accounts for typical moisture load in a compact bathroom. Larger bathrooms have more distinct moisture sources spread across the room, so sizing by fixture gives a more accurate number than area alone.
Does duct length change what CFM I should buy?
Yes. A fan's rated CFM assumes a short, straight duct run. Every extra foot of duct and every 90-degree bend adds resistance and cuts the real airflow, so a long or winding run to the exterior wall or roof may call for a model rated above your calculated minimum.
What happens if my fan is undersized?
Moisture lingers longer after a shower, which raises the risk of mold and mildew on grout, drywall, and ceiling paint over time. Sizing correctly from the start is cheaper than replacing damaged surfaces later.
For more on fan selection and moisture control, see how to choose a bathroom exhaust fan that actually works, how to prevent bathroom mold in the first place, and how to remove bathroom mold and mildew safely.