Tool
Bathroom Exhaust Fan CFM Calculator
An undersized fan leaves steam sitting on the ceiling after every shower. Enter your bathroom's size and get the airflow, in CFM, that actually clears it.
Your bathroom
Your recommendation
Enter your bathroom's dimensions and the recommended fan airflow shows up here.
CFM stands for cubic feet per minute, how much air the fan moves. The standard sizing rule is simple: one CFM per square foot of floor area, with 50 CFM as the floor. Larger bathrooms, over 100 square feet, are sized by what is in them instead, since a big bathroom with a jetted tub makes far more steam than one with just a toilet and sink.
The number matters for mold because a shower dumps a surprising amount of water into the air, and that moisture typically needs 15 to 20 minutes of venting to clear. Run the fan during the shower and keep it going afterward, a timer switch makes this automatic and is one of the cheapest mold-prevention upgrades there is.
Two common mistakes: buying on price and ending up under 50 CFM, and forgetting the duct. A right-sized fan choked by a long, kinked, or crushed duct run moves far less air than its rating. If your mirror stays fogged 10 minutes after a shower, the fan or its ducting is not keeping up.
Guidance, not a spec sheet
This follows the common HVI sizing rules, but duct length, elbows, and local code can change what a given bathroom needs. An electrician or HVAC pro can confirm before you buy.
Already seeing spots on the bathroom ceiling or around the fan? A local pro can tell you whether it is a wipe-away or something deeper.
Find a local mold pro