What Size Exhaust fan Do I Need For My Bathroom?

What Size Exhaust fan Do I Need For My Bathroom?

When installing a bathroom ventilation in a new bathroom remodel or a new construction, the most important spec is the size of the bathroom exhaust fan in CFM or cubic feet per minute. The CFM is the volume of air that will effectively be exchanged via either exhaust (moving stale air out to the bathroom) or intake (bringing fresh air into the space). The ability of your exhaust fan to quickly and efficiently move air depends on the fan's CFM rating which can eliminate many of the comfort issues that are common in bathrooms, including:

• Fogged-up mirrors
• Mold growth on wet surfaces
• Bathroom odors
• Air moisture while showering
• Fumes from strong cleaning agents
• Protect bathroom finishes including metal, wood, and painted surfaces

Here, we will show you three methods used to calculate the size of the bathroom exhaust fan you will need to keep air circulating, eliminate odors, and remove moisture in the air.

3 Ways to Calculate Exhaust Fan Size

Standard Fan Sizing

The most basic calculation for a bathroom that is 100 square feet or under is to assign 1 CFM per square feet of bathroom floor space. This calculation also assumes the bathroom ceiling is 8 feet tall or lower and that you are choosing a product that produces a full exchange of air eight times per hour.

A more precise determination of fan size based on square footage is to calculate the exact square footage of the area and choose an exhaust fan to meet the size of the space.

Here, you will calculate the bathroom square footage:

• Sq. Ft. = room length X room width

So, if your bathroom area is 5’ x 7’ = 35 sq. ft., you should consider over sizing the fan slightly and select a standard exhaust fan with a CFM greater than 35, which would be a 50 CFM bathroom exhaust fan.

Calculation for Large Bathrooms

If you have a large bathroom with a ceiling that is higher than 8 feet, then follow this preferred method of calculation:

• Multiply the width x length x height of the room
• Divide by 60 (minutes in an hour)
• Multiply by 8 (number of air exchanges per hour).

Therefore, for a 15′ wide by 12′ long bathroom with a 10′ ceiling, the calculation would be as follows:

15x12x10 = 1800

1800 ÷ 60 = 30

30×8 = 240 CFM rated vent fan

Using a FAN CFM Calculator

An exhaust fan CFM calculator will allow you to obtain the most accurate sizing for your bathroom fan, because you can adjust the frequency of the desired air exchange. Not only will you obtain the exact volume of air exchange needed in CFMs, the best CFM calculators will also help you choose the right product for roof or wall mounted exhaust fans that provide intake air, exhaust, or both.

Another great tool that is found with many CFM calculators is an extensive list of interior spaces other than bathrooms that require ventilation. These Minute Air Exchange Charts For Commercial and Industrial Applications are a great way for HVAC service professionals to determine the best air change rates or how many times air should enter and exit the space within one hour.