Bathroom exhaust fan CFM calculator
An undersized fan can't clear shower steam, and that moisture becomes mildew. Enter your bathroom size for the recommended fan capacity inCFM (cubic feet per minute).
Evidence-checked. Facts and figures on this page are verified against published standards and government sources (HVI bathroom ventilation guidance (1 CFM/sq ft, 50 CFM min)). Last verified July 2026 — our methodology.
Recommended fan
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For bathrooms up to 100 sq ft, the Home Ventilating Institute rule is about 1 CFM per square foot of floor, with a 50 CFM minimum. For larger or tall bathrooms the fixture method is more accurate: 50 CFM each for a toilet, shower, and tub, and 100 CFM for a jetted tub. Always vent the fan to the outdoors — never into the attic — and run it during the shower plus about 20 minutes after.
Sizing a bathroom fan the right way
The fan's job is to pull shower moisture out before it settles on cold surfaces and grows mildew, so bigger is usually safer than smaller — round up to the next stocked size (they come in 50, 80, 110, 150 CFM and up). Ceilings taller than 8 ft hold more air, so this tool nudges the number up for them. If your bathroom has no window, ventilation is non-negotiable — seehumidity in a bathroom with no window. To confirm the fan actually turns the room's air over enough times an hour, use theair changes per hour calculator, and for whole-room damp, size adehumidifier instead.