How to Determine the Size of an Exhaust Fan for a Bathroom

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By eHow Contributing Writer

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Bathroom ventilation fans provide an exchange of fresh air into the room by pulling odors and bath and shower steam from the room. Today's large bathrooms present an even greater demand for proper ventilation. Good ventilation will also help eliminate mold and mildew. Bathroom ventilation fans are measured in CFM, which is an abbreviation for Cubic Feet per Minute. Different methods can be used to calculate the recommended size of a bathroom ventilation fan. When sizing a ventilation fan for a bathroom, the goal is to produce eight complete air changes within an hour.

Difficulty: Easy

Instructions

Things You'll Need:

Tape measure
Calculator

Step 1

Measure the length of the room. If you are measuring this alone and are having difficulty holding the tape measure in place, break the measurement into two parts, and add them together.
Step 2

Measure the width of the room. Again, break into two measurements if you're having difficulty.
Step 3

Measure the height from floor to ceiling. Round up to the nearest foot for easier calculation.
Step 4

Multiple length times width times height of the wall to determine the cubic feet of the room.
Step 5

Divide the cubic feet by 60, the number of minutes in an hour. Your goal is to achieve 8 air changes per hour, which is why you divide the cubic feet by 60 minutes.
Step 6

Multiply by eight since eight air changes per hour is your goal.
Step 7

Find a fan at your local hardware store that is close to this number; round up to the higher number if possible.

Tips & Warnings

If the room you are measuring is not square or rectangle, break it into sections to get the length and width measurements of those areas. Multiply each area's length times width times ceiling height to get the cubic footage for those areas. Add the areas together to come up with the grand total for the room. The ventilation goal for air changes per hour for a kitchen is 15; it is 6 for any other room in the house.
When installing a long duct run to the outside, it can affect your sizing calculation as well as multiple elbows. If this is the case, you might look at rounding up to the next largest fan size. If you have a steam room, sauna or hot tub, compensate for the additional moisture that these items produce. One other item to keep in mind is a cathedral ceiling. If the ceiling is 8 feet tall at the low end and 10 feet tall at the peak, multiply ceiling height by a factor of 1.25 to compensate.