Bathroom Fan Size Calculator
This free bathroom fan size calculator uses the official Home Ventilating Institute (HVI) sizing rule to recommend the right CFM for your bathroom.
Enter your bathroom's square footage, and for larger bathrooms, which fixtures it has, to get a recommended CFM range in seconds.
This calculator gives a starting estimate based on HVI's published sizing rule. Always check your local building code for any additional minimum CFM or continuous-ventilation requirements.
How This Calculator Works
The Home Ventilating Institute (HVI) publishes two official rules depending on bathroom size. For bathrooms under 100 sq ft, it's simple: 1 CFM of fan capacity for every square foot of floor area. For bathrooms 100 sq ft or larger, square footage alone isn't considered reliable, so HVI switches to a fixture-count method instead — each toilet, shower, or standard tub adds 50 CFM, and a jetted or whirlpool tub adds 100 CFM, since it churns and aerosolizes far more moisture than a standard tub. This calculator applies whichever rule actually fits your bathroom's size automatically, so you don't need to know which one to use.Why Correct Fan Sizing Actually Matters
An undersized fan runs the whole time you shower and still leaves the mirror fogged and the ceiling damp, because it physically can't move the moist air out fast enough. That trapped moisture is exactly what feeds the mold and mildew problems covered throughout this site's Humidity & Air Quality content — a bathroom with a correctly sized fan rarely develops a ceiling mold problem in the first place. Oversizing isn't really a concern here the way it is with AC or a dehumidifier — a bathroom fan that's a little too powerful is just loud, not harmful. If anything, rounding up slightly within reason is safer than rounding down.What This Calculator Doesn't Account For
This tool gives HVI's published sizing recommendation, not a full code compliance check. Local building codes sometimes set their own minimum CFM or require continuous low-speed ventilation rather than just intermittent use. Duct length and the number of elbows between the fan and the outside also affect real-world performance — a fan rated for 80 CFM can deliver meaningfully less once it's pushing air through 15 feet of flexible duct with several bends. If you're replacing a fan that's clearly too weak for a space HVI's rule says should work fine, duct restriction is a common hidden cause worth checking before buying a bigger fan.Frequently Asked Questions
Not really — unlike an oversized AC or dehumidifier, a stronger bathroom fan just clears moisture faster and runs louder at full speed. Many fans also have adjustable speed settings, so a little extra capacity is rarely a problem.
A jetted or whirlpool tub agitates the water, which releases far more water vapor into the air than a standard bath where the water just sits still. HVI's rule accounts for that directly.
Possibly, but check duct length and condition first — a correctly sized fan pushing air through a long or crushed duct run can perform like a much weaker one. Compare your fan's rated CFM to this calculator's result before assuming you need a bigger unit.
No — for bathrooms under 100 sq ft, one fan sized to the room's total square footage handles both fixtures together. The fixture-count method only applies once the room crosses 100 sq ft.
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This calculator is for general informational purposes and provides an estimate based on HVI's published sizing rule. Always confirm requirements against your local building code.
