February 7


Can A Bathroom Fan Be Too Big? Bathroom Fan Sizes

By Robin Hatch

February 7, 2022

The NEC (National Electrical Code) and local building codes mandate bathroom vent fans in any bathroom that does not have a window that can be opened to allow ventilation. They are necessary for some areas, even if windows are present.

A vent fan serves two functions: It can eliminate scents and hot, damp air. Mold grows in hot, wet air, and smells are generally unpleasant.

Even if the fan meets the minimum criteria, installing a weak, undersized fan is usually never worth the effort. It’s, therefore, essential to understand how vent fans are sized and select one with the appropriate capacity for your space. So can a bathroom fan be too big?

What is the standard fan size, and how do you choose the perfect fan for your bathroom? In this article, we’ll answer all of these puzzles.

Read on to learn more. 

What Is The Standard Fan Size?

The volume of air that a bathroom vent fan can move is measured in cubic feet per minute, or CFM. Standard fan sizing applies to bathrooms of 100 square feet or less. The general rule is that you need at least one CFM per square foot of room size.

Multiply the length by the width to get the square footage of your bathroom. If your bathroom is 6 feet wide and 9 feet long, the square footage is 54. As a result, it should have a fan with a CFM rating of at least 54. But before you go shopping, there are a few things to think about.

  • First, it’s a good idea to slightly oversize the fan. In our 54-square-foot example, for example, a 60 CFM fan is a smart idea for good measure.
  • Second, if your bathroom has a jetted tub, different rooms, or alcoves, you may want to consider purchasing more than one fan.
  • The minimum allowable fan size is 50 CFM; therefore, even if your bathroom is only 42 square feet, you’ll still need a 50 CFM fan.

An element to consider when sizing a vent fan is duct size and length. The majority of 50 CFM fans will work nicely with a 4-inch round duct. However, when the CFM of the fan increases, the duct size must be adjusted to a 5- or 6-inch circular duct.

Estimating optimum duct size for various length runs and combinations is quite technical. Nonetheless, the instructions that come with the fan will clarify the requirements. Just make sure there’s enough room where you’re placing the fan for the ducting.

Also, driving too much air through a too-small duct will cause the fan to work too hard and offer insufficient ventilation.

Fan Size For Larger Bathrooms

Exhaust fans for bathrooms larger than 100 square feet can be sized based on the number of fixtures in the room. Add up the required CFM ratings for all of the fixtures to determine using this formula:

  • 50 CFM in the bathtub.
  • 100 CFM jetted tub.
  • Shower: 50 CFM.
  • 50 CFM in the toilet

If your bathroom only has a shower and a toilet, a 100 CFM fan is required. Yet, a bathroom with a jetted tub, toilet, and shower needs a 200 CFM fan.

Another technique of calculating is beneficial for large rooms with high ceilings (above 8 feet). Multiply the square footage by the ceiling height, divide by 60 (minutes in an hour), then multiply by 8 in this technique. For a 120-square-foot room with a 10-foot ceiling, for example:

  • 120 x 10 = 1,200, 1200/60 = 20. 

Why You May Need A Second Fan

If your bathroom includes a toilet or shower section that is separated from the rest of the room by a door, it is a great idea to add a second exhaust fan for that area. A 50 CFM fan is sufficient if the enclosed space is modest.

Otherwise, do the necessary estimate based on the size of the room. Just keep in mind that the fan needs airflow to function correctly. If the enclosure door is closed and there is no space at the bottom, the fan will be deprived of “makeup” air and may function inadequately.

Is Higher CFM Better For Bathroom Fans?

The larger the bathroom, the higher the CFM rating you’ll require. A fan must have a CFM rating high enough to replace the air in your bathroom eight times per hour.

What Is An Ideal CFM For A Bathroom Fan?

Generally, ventilation rates of eight air changes per hour are recommended. This equates to one CFM per square foot of bathroom area in most cases.

A 70 CFM fan, for example, would be required in a 7′ x 10′ bathroom. For restrooms of 50 square feet, a minimum CFM rate of 50 is suggested.

How To Choose The Perfect Fan For Your Bathroom

When purchasing a bathroom fan, you should first evaluate the size of your bathroom, followed by additional features such as lighting, heat, and sensors that turn the fan on automatically. Finally, because sound levels can vary greatly, think about how much noise you can tolerate.

Noise Level

Bathroom fans that sound like jet engines might significantly interfere with your bathroom’s tranquillity. When shopping for a fan, you should consider the noise level and choose a quieter type.

The sound of a bath fan is measured in sones. A sone is a measurement of sound based on how the typical listener perceives it. In a quiet kitchen, one sone is about equivalent to the gentle hum. That sound would be doubled with two sones, and so on.

Choose a model with a sone rating of two or less if you want a quiet bathroom fan. White noise is produced more by fans rated at three and four sones. Avoid fans with a decibel rating of five or above.


When purchasing any home appliance or system, consider efficiency first. Exhaust fans, fortunately, have become more efficient than ever before.

Bathroom fans that are Energy Star approved use 70% less energy than non-Energy Star models. This surely helps to keep energy bills down. They must also fulfill requirements for minimal sound emission, low wattage consumption, and powerful but effective static pressure performance.

To get the most bang for your budget, use an Energy Star bathroom exhaust fan. You may also get a fan with a motion and humidity sensor to make sure it only operates when it’s needed.


Most bathroom fans are mounted on the ceiling, although certain types may also be mounted on the wall. The units come with built-in duct adapters that connect to the ductwork in your home. Instead of venting air into an attic, you should place your fan to vent air outside your home through your ductwork and the nearest soffit.

Venting air into an attic moves warm, wet air to another part of your house, where moisture issues might still arise. When choosing a bathroom fan, think about where you want it to go: on the ceiling or the wall. Then, select a model with the right size duct adapter (measured in inches).

Other suggestions

Existing Wiring: Consider placing a fan with light over an existing light fixture for easy wiring and convenience.

Shower + Tub Fan Safety Ratings: If you’re going to place the fan above a shower/tub combo, ensure it’s rated for this position (typically, UL-listed).

Installation by an expert: If you require assistance, consider hiring an HVAC specialist to install your fan or an electrician to complete the wiring requirements.

Ideal Fan Features

Today, bathroom ventilation fans have various functions that make them more convenient and efficient. So you may choose a bathroom fan depending on the following features. 

Integrated Lighting: Many models come with built-in lights that can be used to replace or supplement existing fixtures. Night lights are even included in certain models for mild illumination during the night.

Heaters: If you live in a chilly region, fans with built-in heaters might offer extra warmth.

Humidity + Motion Sensors: Humidity and motion sensors help the fan run more efficiently and save energy. A humidity sensor monitors the humidity in your bathroom and turns on the fan when the humidity climbs over a specified level, then turns off when the humidity falls below that level. A motion sensor detects it and switches on the fan or light alone when someone enters the bathroom.

Controls: Fans with many functions often have different controls for each function, or you may operate them all at the same time. You’ll also get a separate wall switch to regulate these features easily.


Choosing the right fan for your bathroom is essential to keeping your bathroom and the air inside it clean. It also helps keep you and your family safe. We hope this article was helpful and will assist you in finding the one which best suits your home and your needs.

Robin Hatch

About the author

For over a decade, Robin has been a real estate agent, interior design specialist, and mother. Through her trials and tribulations, she wanted to create the perfect website to help you save money and make your home look beautiful.

{"email":"Email address invalid","url":"Website address invalid","required":"Required field missing"}