Bathroom Exhaust Fans: The Published CFM Rule and the Sone Tradeoff

Bathroom fans are bought for code and ignored by morning. Yet the box publishes two numbers that fix this: CFM (air movement) and sones (noise). Choosing correctly on both is the difference between mold prevention and decoration.

The quick verdict

• The published industry rule: 1 CFM per square foot of bathroom, minimum 50 CFM — more for tall ceilings and deep tubs.
• Published 1.0 sone or less is quiet enough that people use it; a 4-sone fan is a mold generator with a light.

Winner by scenario

• Standard 8×10 bathroom: Published 80–110 CFM, low-sone model — Meets the published sizing rule while staying quiet enough for daily showers.
• Windowless bathrooms: Published CFM plus humidity sensing — Sensing models run until the published humidity target clears, removing human forgetfulness.
• Retrofits without attic access: Published ceiling-insert replacements — Like-for-like published housing sizes avoid drywall surgery.

What not to overpay for

• Bluetooth speakers in fans whose published sone rating climbs with the extras.
• CFM ratings without published sone figures — quiet and powerful must both be published to be believed.

The tradeoff, stated plainly

High published CFM usually costs more noise (or more money for engineered quiet). Size the room honestly first; then spend the savings on sones.

FAQ

How long should the fan run?

Published guidance suggests 20 minutes post-shower; timer or humidity-sensing published models automate it.

Can one fan serve two bathrooms?

Published code varies; generally separate fans per room perform better and pass inspection more easily.