The two accepted ways to size a bathroom exhaust fan, and why real-world performance runs below a fan's rated number.
For a smaller bathroom
For a bathroom up to 100 square feet, the sizing guidance is 1 CFM of airflow per square foot of floor area, which works out to roughly eight air changes an hour in a typical bathroom, with a 50 CFM floor regardless of how small the room is.[1]
To use this rule, multiply the room's length by its width in feet to get square footage, then size the fan to at least that same number in CFM.
For a larger bathroom
Above 100 square feet, sizing shifts from floor area to fixtures: 50 CFM for each toilet, shower or standard tub, and 100 CFM for a jetted tub, added together for the room's total.[1]
A larger bathroom with a separate shower, tub and toilet can add up to a fan rating well above what the square-footage rule alone would suggest, which is why the fixture method takes over once a room passes the 100-square-foot mark.
Why the rated number isn't the real number
Duct length, bends and termination hardware all reduce a fan's output below its rated CFM in real installations, commonly by 15% to 30%, so a fan sized exactly to the calculated minimum often underperforms once it's actually ducted through a wall or roof.[1]
The practical fix is to size 20% to 30% above the strict calculated minimum, giving the installed system room to still clear the room's moisture even after duct losses.[1]
Putting it into a renovation plan
If your bathroom is being reconfigured, decide on fan size and duct routing before the wall and ceiling are closed up, since a duct run added later is far more disruptive than one planned alongside the rest of the framing.
A shower renovation is also a natural point to upgrade a fan that was undersized to begin with. A fan struggling to clear steam is a common reason a bathroom shows mildew on the ceiling or around the shower trim even when the shower itself was recently redone.
A worked example
A 7-by-10-foot bathroom, 70 square feet, falls under the smaller-bathroom rule, so the straight calculation calls for a 70 CFM fan at minimum, comfortably above the 50 CFM floor.[1]
Adding the recommended 20% to 30% buffer for duct losses pushes that toward 85 to 90 CFM as the fan rating actually worth buying, not the bare 70 CFM the floor-area math produces on its own.[1]
Compare that against a bathroom with a separate shower and toilet using the fixture method instead: 50 CFM for the shower plus 50 CFM for the toilet totals 100 CFM before any buffer, already above what the smaller worked example called for.[1]
Questions people ask
Does a bigger fan always mean a noisier bathroom?
Not necessarily; fan noise is rated separately in sones, and a larger, well-selected fan can run quieter than an undersized one working harder to move the same air.
Should the fan run during the shower or after?
Both; running it during the shower and for a stretch afterward clears the bulk of the moisture rather than letting it linger and settle into grout, drywall or a ceiling above.
Does a window substitute for an exhaust fan?
In some code jurisdictions an operable window is treated as an alternative to mechanical ventilation, but a fan works regardless of weather or whether anyone remembers to open the window.
Is it worth buying a fan larger than even the buffered recommendation?
Beyond a reasonable buffer for duct losses, an oversized fan mainly adds noise and pulls conditioned air out of the house faster without meaningfully improving moisture control.