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How To Calculate Fan Size for Bathrooms

To properly size a bath fan, you can follow the recommendations from the Home Ventilating Institute (HVI) guidelines:

Fan Sizing for Bathrooms Under 100 SQ. FT.
The standard rule for bathrooms 100 square feet or smaller is one CFM per square foot of the bathroom. To find the square footage multiply the length of the bathroom by the width.

Example: 10’ x 10’ bathroom = 100 square feet.
Result: You will need a 100 CFM fan for this bathroom.

Fan Sizing for Bathrooms Over 100 SQ. FT.
Ventilation for baths over 100 square feet is calculated based on the fixtures in the bathroom. Allow 50 CFM for each standard toilet, shower, steam shower or tub. Whirlpool, garden and hot tubs are calculated at 100 CFM each. Add the CFM for all fixtures and you’ll have the total CFM required for the bathroom.

Example: Step 1: Calculate Bathroom Size: 20’ x 10’ Bathroom = 200 square feet.
Step 2: 1 Toilet +1 Shower +1 Tub = 150 CFM
Result: You will need a 150 CFM fan for this bathroom.