What Size Bathroom Fan Do I Need

As many of you know I run an electrical contracting company called Electrical Service Pros and I’m constantly being asked to install or replace bathroom fans. Some customers want to purchase their own exhaust fans and want us to install them. I always ask them if they purchased the right size. Most customers are unaware that bathroom fans need to be sized according to the area of the bathroom.

How to Determine the Right Size Bathroom Fan

Bathroom exhaust fans are rated by the cubic feet per minute CFM of air movement. The HVI (Home Ventilation Institute) recommends that 8 air changes per hour are required to properly vent the bathroom. For most bathrooms (those with an eight-foot ceiling) that would equate to 1cfm per square foot. So a 10′ by 8′ bathroom would need an 80 CFM fan.

What If My Ceilings are over 8′

If your ceilings are over 8′ we will need to figure out the cubic feet of the room. We do this by multiplying the length by the width and by the height. So a 10′ by 10′ bathroom with 10′ ceilings would be 1000 cubic feet. Now we multiply by 8 (the number of air changes per hour we will need) which gives us 8000 cubic feet per hour and the divide by 60 (minutes in an hour). That gives us 133.3 CFM. Obviously they do not make an exhaust fan that size so we would round up to a 150 CFM fan.

10’x10’x10′ bathroom = 1000 cubic feet

1000 cubic feet x 8 air changed per hour = 8000 cubic feet

8000 cubic feet / 60 minutes = 133.33 CFM

150 CFM Bathroom Fan will be required.

Sample Bathroom Dimensions and Required Fans

Rooms with 8′ ceilings:

6′ x 8′ = 50 CFM fan

8′ x 8′ = 80 CFM fan

10′ x 10′ = 100 CFM fan

Rooms with 10′ ceilings:

6′ x 8′ = 80 CFM fan

8′ x 8′ =100 CFM fan

10′ x 10′ = 150 CFM fan

What Can I Do If My Bathroom Fan is Undersized

If you bathroom exhaust fan is sized correctly it will provide 8 air changes per hour or every 7.5 minutes. Just based on that, when you shut off your shower the fan still needs to be running for approximately 8 minutes to properly vent the room. If your fan is already undersized and you do not wish to replace it there may be options.

The 1st option would be to install a fan timer. This is mounted in place of the switch and allows your fan to run for a specified length of time. Typically, 5-10-15-30 minutes.

The 2nd option would be to install a humidity sensing fan control. A switch that comes on when the humidity gets to high and will shut off when the moisture is gone.

I hope this information is helpful sizing for fan. If anyone has non typical dimensions or needs additional help please leave a comment and I can give you my recommendation.