Bathroom Ventilation Fan CFM Calculator
By Bathroom Size
Size a bath fan using the 1 CFM per sq ft rule
By Fixture Count
Size a bath fan for larger bathrooms using the fixture method
Formula
Small Bath: CFM = Sq Ft x 1 (min 50) | Large Bath: CFM = (Toilets + Showers + Tubs) x 50 + 50 if jetted tub
Frequently Asked Questions
What size bathroom fan do I need?
For bathrooms up to 100 sq ft, use 1 CFM per square foot (minimum 50 CFM). For larger bathrooms, add 50 CFM for each toilet, shower, and bathtub. Add another 50 CFM if you have a jetted tub. A standard 8x10 bathroom needs an 80 CFM fan.
Should I run the bathroom fan during a shower?
Yes, run the bathroom fan during and for 20-30 minutes after a shower to properly remove moisture. A humidity-sensing fan switch can automate this. Proper ventilation prevents mold growth, paint peeling, and structural moisture damage.
Can a bathroom fan be too big?
A slightly oversized fan is better than undersized. However, a drastically oversized fan can depressurize the bathroom, potentially causing backdrafting from water heaters or furnaces. Stay within 20% of the calculated CFM for best results.
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