Proper Bathroom Exhaust Fan Sizing & Bathroom Vent CFM Calculator

Bathroom vent GFCI protection is required for installations over the shower and tub areas.


Bathroom vent CFM calculator uses 3 stage formula located below. It is based on HVI (Home Ventilating Institute) guidelines that recommend approximately 8 room air exchanges per hour  for a bathroom under 100 square feet of area.

CFM – cubic feet per minute – general term used to specify the amount of air that the bathroom fan  is able to move within one minute of its operation.

  • First you have to measure your bathroom dimensions to calculate room volume (length x width x height of the room = … feet³)
  • Divide the number of … feet³ by 60 minutes (fans are sized based on the number of recommended air exchanges per hour / 60 minutes)
  • Multiply the result by 8 (eight is the recommended number of room air exchanges per hour)

Any result smaller than 50 CFM would still require minimum 50CFM exhaust fan (building code requirement). HVI ventilation rate recommendation for bathrooms larger than 100 square feet is slightly different and based on the number and type of plumbing fixtures within the bathroom area.

Each fixture (toilet, shower stall , bath tub) requires 50 CFM, Jacuzzi / hydro spa / whirlpool – 100 CFM. All you need to do is add number of fixtures and multiply by the required CFM.

Example of a calculation for bathrooms with over 100 square feet of area:

Toilet (50 CFM) + shower stall (50 CFM) + Whirlpool (100 CFM) = 200CFM bathroom exhaust fan.


Building code does not specify bathroom vent locations but in any size bathroom it is the most logical to install it over or as close as possible to the plumbing fixture that requires it. You can also install two or more individually controlled bathroom exhaust fans over different areas (with combined CFM’s equal to your calculation).

This would be the most efficient way to control moisture and bathroom odors (enclosed toilet rooms require separate exhaust fans).

Remember: bathroom vent GFCI protection is required for installations over the shower and tub areas.

Based on the final results of the bathroom vent calculator or bathroom exhaust manual calculations – pick the bathroom exhaust fan and install it in the most appropriate location.

1 Comment
  1. sergio b. diasana, sr says

    it is a good help for those who is interested to know the technical aspects of ventilation system for human comfort.

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