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How many Cubic Feet Per Minute (CFM) Should
My Bathroom Exhaust Fan Produce?

Q. "Could you please tell me the CFM (cubic feet per minute) that is needed to properly ventilate a bathroom approximately. 6' X 8' x 8'. There is no window to help out the ventilation and my wife is threatening to send me to the closest gas station!" Submitted by DM from Winnipeg, Manitoba, Canada

A. The minimum ventilation required for a bathroom is 12 complete changes of air per hour.

The formula for optimum bathroom ventilation, measured in cubic feet of air moved per minute, is...

Total Volume of Air in the Room ÷ 5 = Minimum Recommended CFM

The "magic number 5" in this formula is based on 12 changes per hour, divided by 60 minutes/hour to give us the number of changes per minute.  To apply this to your bathroom, multiply the width by the length by the height to determine the total cubic feet...

Width × Length × Height = 6ft. × 8ft. x8ft. = 384 cubic feet

Divide the volume of your bathroom by 5 to find out the recommended CFM...

384 cubic feet ÷ 5 = 76.8 CFM or Cubic Feet per Minute

Based on personal experience, I would recommend purchasing the best quality fan you can afford that exceeds your computed CFM value. Don't skimp, since this should be a one-time purchase for most homeowners.  Better quality exhaust fans offer higher CFM's and better motors meaning a longer service life, a quieter bathroom... and a happier marriage!!

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