Last Updated on by
Learn how to calculate CFM for a bathroom exhaust fan! Having the right size for your bathroom is very important! Bathrooms with higher ceilings and larger dimensions require more powerful fans to keep moisture and humidity at appropriate levels.
Bathroom exhaust fans, or extractor fans, come in variety of sizes. Knowing what size bathroom fan you need is easy to calculate. All you need is a tape measure and a calculator. “What size bathroom fan do I need Cheryl?” Here’s how to size a bathroom fan!
IMPORTANT: An extractor fan MUST be routed to the exterior of your home. Make sure your contactor, doesn’t take a short cut and run all that moisture into the attic! Read this to understand why.
How to Size an Exhaust Fan for a Bathroom
- Measure the dimensions of your bathroom. Use a tape measure to measure the length, width, and height of your bathroom. If you have an unusually shaped bathroom, it might be easier to draw out your floor plan and label each wall to keep track of your dimensions.
- Next, use your dimensions to calculate the total volume of your bathroom. For standard square or rectangular bathrooms, you can use the formula Length X Width X Height = Volume.
- Calculate how many CFM your bathroom fan needs by dividing the number from the step #2 by 7.5. This is how often your extractor fan should be able to replace the bathroom with fresh air. (More on this below)
A Quick Example
For a standard 10ft by 10 ft bathroom with an 8 ft ceiling, you will need an exhaust fan capable of 106.67 CFM. Here’s how we get this number:
(10ft * 10ft* 8ft)/7.5 = 106.67 CFM
Bathroom Fan Exhaust Size Calculator
The formula to calculate bathroom exhaust fan size reads as follows:
Cubic Volume/7.5= CFM Necessary
Why You Need to Have the Proper Size Extractor Fan
Bathroom extractor fans are mandatory in any bathroom without a window and still highly recommended in bathroom with windows. The exhaust fan removes moisture and humidity from your bathroom. Moisture and humidity can significantly alter the life expectancy of appliances.
- Exhaust fan must be strong enough to restore the bathroom with fresh air every 7 1/2 minutes or 8 times an hour. This is the industry standard.
A fan that is too small will not effectively remove moisture from the room. This can lead to serious problems including, warped cabinetry, fogged mirrors, mold, and/or mildew growth. Poor-ventilation is one of the main reasons why many people say MDF is not a good material in the bathroom. Although solid oak is better, MDF is fine when you have the right size exhaust fan. It’s especially important when you have MDF cabinets.
What Does CFM Stand For?
CFM stands for Cubic Feet Per Minute. It measure the amount of air being moved per second. Here’s a definition from Webopedia.
“A measurement of the velocity at which air flows into or out of a space. The CFM measurement is used in reference to a computer’s cooling system, and more typically in reference to an air-cooling system that is supporting overclocking.”
Most bathroom exhaust fans come with a rating of between 50CFM-110CFM with larger commercial units offering higher capacities. The rating should be listed somewhere on the packaging or stores sometimes organized them based on CFM.
If you have any other questions, you can leave a comment below. Otherwise, I also recommend reading the other articles below as they also pertain to moisture and humidity in the bathroom.
Bathroom Moisture and Humidity Related:
- Why You Have to Run Your Bathroom Exhaust Fan Through the Roof
- How to Get Rid of Shower Mold
- Best Material for a Bathroom Vanity
About the Author
Cheryl Khan is a designer and writer at Tradewinds Imports, an online speciality site dedicated to fine bathroom furnishings. She has extensive knowledge about all the finer details that go into planning the perfect bath renovation project and is an expert on all things bathroom! Send her your bathroom Q’s on twitter @SuperInteriors!