Bathroom Exhaust Fan Calculator
Estimate bathroom exhaust airflow from room volume and target air changes, then account for duct loss, fan quantity, energy use and cost.
Default project preview
85 CFM
Recommended fans
1
Room volume
640 ft³
Effective airflow
88 CFM
Estimated actual ACH
8.3
Calculator
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Enter the project details and calculate a rounded material estimate.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Required airflow = room volume × target ACH ÷ 60 for CFM, or room volume × ACH for m³/h. Fan count uses rated airflow after the duct-loss allowance.
FAQ
Bathroom Exhaust Fan Calculator questions
Why is installed airflow lower than the fan rating?+
Ducts, elbows, grilles and exterior terminations create resistance that can reduce delivered airflow.
Should the fan run after a shower?+
Post-run time can help remove remaining moisture, but controls and duration depend on the room and local guidance.
Can the fan discharge into the attic?+
Generally exhaust should terminate outdoors through an approved route, not into an enclosed building cavity.