Jonathon Jachura is a two-time homeowner with hands-on experience with HVAC, gutters, plumbing, lawn care, pest control, and other aspects of owning a home. He is passionate about home maintenance and finding the best services. His main goal is to educate others with crisp, concise descriptions that any homeowner can use. Jon uses his strong technical background to create engaging, easy-to-read, and informative guides. He does most of his home and lawn projects himself but hires professional companies for the “big things.” He knows what goes into finding the best service providers and contractors. Jon studied mechanical engineering at Purdue University in Indiana and worked in the HVAC industry for 12 years. Between his various home improvement projects, he enjoys the outdoors, a good cup of coffee, and spending time with his family.
Casey Daniel is a writer and editor with a passion for empowering readers to improve their homes and their lives. She has written and reviewed content across multiple topics, including home improvement, lawn and garden care, sustainability, and health and wellness. When she’s not reviewing articles, Casey is usually playing board games, repainting her bathroom, or quilting.
April 6, 2024 by Jonathon Jachura
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Bathroom vents are crucial for moisture control and preventing mold growth. These fans remove humid air, but undersized fans are ineffective. Proper ventilation is especially important if you’re planning a bathroom remodel.
I’ll provide step-by-step instructions to measure your bathroom below. With the right CFM, your fan will remove moisture and keep your bathroom walls mold-free.
CFM stands for cubic feet per minute. This is the volume of air a bathroom vent fan can move. The higher the CFM, the more air the fan circulates out of the bathroom.
The two key factors that determine the necessary CFM are:
As a general guideline, you need at least 50 CFM for bathrooms up to 100 square feet. Add 1 CFM for each additional square foot. For example, a 150-square-foot bathroom would need at least 100 CFM. Note that CFM guidelines may vary by your local building code.
Undersized fans are ineffective and can lead to excess moisture, causing warped cabinets, peeling paint, and mold growth. Oversized fans are louder and use more energy.
Follow these steps to accurately measure your bathroom’s dimensions before using the calculator:
Measure carefully for the most accurate sizing result. Calculate CFM and vent fan size separately for each bathroom in your home.
Use the CFM recommendations from our calculator to select an appropriately sized vent fan for your bathroom. Refer to the sizing chart below as a guideline.
| Your Bathroom Size (sq. ft.) | Minimum Vent Fan Size (CFM) |
| Up to 50 | 50 |
| 50–100 | 50–80 |
| 100–150 | 100–150 |
| 150–200 | 100–130 |
| 200–300 | 130–170 |
| 300–400 | 170–210 |
This chart provides the CFM range you should look for based on your bathroom’s square footage. Choose a vent fan toward the upper end of the range to make sure it can adequately remove moisture.
Today’s Homeowner TipsAlso, pay attention to the vent fan’s sones rating, which measures sound levels. The lower the sones, the quieter the fan will be. Look for fans with sones around 1.0 to 1.5 for relatively quiet operation.
Proper installation is key to maximizing your bathroom vent fan’s effectiveness. Follow these best practices:
An HVAC professional can install your bathroom vent fan properly for optimal performance. Understanding duct replacement costs can help you budget for any ductwork needed.
Operating your vent fan properly is just as important as installation. Be diligent about running your bathroom vent fan to reap the full moisture-control benefits. Here are some tips:
Proper CFM rating, installation, and operation will optimize your bathroom fan to maintain the healthy indoor air your home needs.
Using a bathroom vent fan CFM calculator takes the guesswork out of selecting an appropriately sized fan for your bathroom’s specific square footage. Undersized fans cannot handle the humidity load, while oversized fans are unnecessarily loud and energy-consuming.
The biggest benefit of a CFM calculator is that it provides a simple way to determine the minimum capacity you need based on objective measurements of your actual bathroom.
While the calculator provides a useful starting point, consider the amount of moisture generated in your bathroom based on features like jetted tubs or large multi-head showers. Such high-moisture bathrooms may need supplemental fans or a CFM rating at the higher end of the recommended range.
CFM stands for cubic feet per minute. It is calculated by multiplying the vent fan’s airflow (in feet per minute) by the cross-sectional area of the vent duct (in square feet). This produces the volume of cubic feet exhausted per minute.
The exhaust vent should terminate at least 3 feet away from windows, doors, or other openings in your home. This prevents exhausted moisture from re-entering.
Yes, all bathrooms should have an exhaust fan because of the humidity produced by hot showers and baths. Moisture needs to vent outside rather than lingering and accumulating.
No. After showering or bathing, bathroom fans only need to run long enough to clear excess moisture. Continuous operation wastes energy. Use a timer or humidity sensor to use the fan only as needed.
Yes, you can install a unit with a combined exhaust fan, heater, and light. The heat helps dissipate airborne moisture while the fan exhausts it outside. Just make sure the CFM is still properly sized for the bathroom.
Editorial ContributorsJonathon Jachura is a two-time homeowner with hands-on experience with HVAC, gutters, plumbing, lawn care, pest control, and other aspects of owning a home. He is passionate about home maintenance and finding the best services. His main goal is to educate others with crisp, concise descriptions that any homeowner can use. Jon uses his strong technical background to create engaging, easy-to-read, and informative guides. He does most of his home and lawn projects himself but hires professional companies for the “big things.” He knows what goes into finding the best service providers and contractors. Jon studied mechanical engineering at Purdue University in Indiana and worked in the HVAC industry for 12 years. Between his various home improvement projects, he enjoys the outdoors, a good cup of coffee, and spending time with his family.
Casey Daniel is a writer and editor with a passion for empowering readers to improve their homes and their lives. She has written and reviewed content across multiple topics, including home improvement, lawn and garden care, sustainability, and health and wellness. When she’s not reviewing articles, Casey is usually playing board games, repainting her bathroom, or quilting.
