- 1). Calculate the number of square feet in your bathroom by multiplying the length times the width. If it is over 100 square feet, skip to step three.
- 2). Use the bathroom's square footage to determine the size of the bathroom fan if the size is between 50 and 100 square feet. For example, if the bathroom is 70 square feet, you need a bathroom exhaust fan with a minimum size of 70 CFM. If your bathroom is 50 square feet or less, get an exhaust fan of at least 50 CFM.
- 3). Total the number of toilets, showers, and bathtubs in the bathroom if the square footage is over 100. Multiply this number by 50 to determine the size of your fan. Then add 100 for each whirlpool. For example, if you have a shower, toilet, and whirlpool, your fan would need to be at least 200 CFM.
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