How To Size A Bathroom Exhaust Fan (The Best Method?)

Is it time to install a brand-new bathroom exhaust fan?

Is this the first time you’re installing an exhaust fan?

This is a common requirement in modern homes and it’s important to understand what works and what does in advance.

In this guide, you will learn…

how to size a bathroom exhaust fan
  • How to prepare for a new bathroom exhaust fan
  • How to size a bathroom fan
  • Mistakes to avoid while installing a new fan
  • How to customize the installation process for your property

Follow these steps and your bathroom exhaust fan size will work like a charm!

Read Also: Top 5 Best Bathroom Exhaust Fans With Humidity Sensors (2019 Review)

What Size Bathroom Fan Do I Need?

According to modern research, a fan’s ability to “depressurize” a room is dependent on its size and how well it’s installed. This demonstrates the importance of taking preparations seriously as a property owner.

The average bathroom exhaust fan is designed to meet NEC (National Electrical Code) standards. It’s not only equipped to meet electrical codes, but local building codes too. This is why it’s smart to start preparations beforehand to determine what’s required during the sizing process.

Take a look at where the bathroom exhaust fan will be installed, how it will remove moisture, and how much energy will be used throughout the day.

This information is helpful in determining which bathroom exhaust fan is ideal for the room once it’s been installed.

Remember, when a weaker bathroom exhaust fan size is installed, it will fail to handle moisture and/or odors present in the room. This makes the entire unit useless and a wasted investment. Make sure everything is ready for a brand-new exhaust fan before beginning the sizing process.

This includes assessing the room’s size, where the electrical connectors are, and what capacity will work best.

under-sized bathroom exhaust fans

How To Measure For Bathroom Exhaust Fan Sizing?

The next step is to begin measuring the bathroom.

A tape measure is required to measure the bathroom thoroughly. This includes measuring the height (floor to ceiling) and the length of each wall. If the bathroom is rectangular, these numbers will be useful during the calculation process. You can insert these numbers directly into the CFM formula.

What if the bathroom isn’t shaped like a rectangle?

If the bathroom is oddly shaped, it’s best to sit down and calculate the bathroom’s total volume. This formula is length x width x height.

Measure the Floor

While measuring the floor, it’s recommended to focus on the edges of each room to avoid missing a portion of the bathroom. This often happens when a bathroom isn’t shaped like a square or rectangle.

Start by measuring right at the base of the wall (corners). You want to run the tape measure flat along the floor from one end to the other. Remember to do this in straight lines to avoid taking the wrong measurement. It’s important to note, measuring the walls is not the same as the floors in many cases. This is why separate calculations are required to determine whether or not these numbers are similar.

If the flooring is spread across an oddly shaped bathroom then it’s time to break the space into smaller shapes. Measure each square and add the numbers together. This will lead to the total floor area in your bathroom.

One of the common mistakes property owners make is to ignore the sink, toilet, and bathtub areas. These are a part of the bathroom, which means they need to be accounted for in the final calculation. Include them in the measurements instead of going around.

Read Also: Venting a Bathroom Fan Into An Attic (My Detailed Guide)

How Do I Combine the Measurements?

The official air quality code by the US government states, all quality control tests are designed to examine the total volume inside a room. This is used to determine whether or not the air quality will remain effective with the room’s built-in ventilation setup.

Keeping this in mind, it’s important to begin working on the measurements.

You’ll already have all of the numbers in hand (i.e. length, width, height). It’s time to use these numbers for the right bathroom fan sizing.

Start by taking the bathroom’s floor area (total floor area) and multiplying it by the height (floor to ceiling).


Let’s assume a bathroom is 100 square feet (total floor area) and has a ceiling height of 10 feet.

You will multiply 100 x 10 to get the total volume. In this example, the total volume would be 1000 cubic feet.

What if the ceiling is angled or isn’t built in a straight line?

In such situations, it’s highly recommended to find the average ceiling height by measuring the highest and lowest points.

Average ceiling height = (Highest Ceiling Point + Lowest Ceiling Point)/2

With this number, you can then figure out the total volume of a bathroom.

How To Pick A Final Bathroom Exhaust Fan Size?

It’s now time to size the bathroom fan CFM.

The sole purpose of a bathroom exhaust fan is to remove moisture. As a result, the unit is rated for the amount of air it can move out of a room. In this case, the measurement is noted as CFM (Cubic Feet per Minute).

Bathroom exhaust fans come in various sizes based on a bathroom’s total volume area. In general, the average bathroom exhaust fan is labeled as a “standard fan,” which works for any room under 100 square feet.

With this in mind, a property owner should aim for 1 CFM per square foot or approximately 100 CFM for a standard bathroom.

However, it’s important to get an accurate number while sizing a bathroom exhaust fan. You don’t want a situation where the fan is underpowered and doesn’t clear the air as needed.

To get the right bathroom fan sizing, you will have to use the measurements taken in the previous steps.

The bare minimum is to go for a fan that’s in line with the square footage of a bathroom.

For example, if the walls (length x width) were measured out to be 100 square feet then the bathroom exhaust fan should be at least rated at 100 CFM. You can use the total volume to get an even more accurate representation of the bathroom’s size.

A general rule of thumb is to go above this number instead of trying to match it.

This means if the room is measured to be 100 square feet, you should be going for something that’s at least 110+ CFM. This ensures any unaccounted space is taken care of once the bathroom exhaust fan is set up and ready to go.

In some situations, you may also require a secondary fan. This can happen if the bathroom is split into two sections.

The average bathroom exhaust fan size can vary in terms of its CFM. However, the smallest a person can go with is a 50 CFM fan.

What about a larger bathroom?

If the bathroom is measured to be above 100 square feet, a powerful bathroom exhaust fan is mandatory. This is the only way to make sure the air is appropriately managed throughout the day. You will want to take into account the number of fixtures in the bathroom (i.e. bathtub, shower, toilet, sink) before choosing the right fan.

A common tip is to add 50 CFM with these additional fixtures to make sure the bathroom is efficiently managed. Otherwise, the bathroom exhaust fan may not do well as soon as it’s installed.

For example, let’s assume a bathroom is measured to be 150 square feet. This means it needs a bathroom exhaust fan rated at 160+ CFM. However, what if there is a jetted tub and shower inside? In this case, you would add 50 CFM to the rating meaning it’s best to buy a 210+ CFM exhaust fan.

A secondary formula you can follow is to multiply the ceiling height by the square footage and dividing by 60 minutes. Take this number and multiply by 8 (recommended air exchanges per hour).

Let’s use this formula in an example.

If the room is measured at 140 square feet with a ceiling height of 10 feet, you will get the following answer:

140 square feet x 10 feet = 1,400

1,400 / 60 minutes = 23.33

23.33 x 8 = 186.64 CFM

In this case, you would round up and buy a bathroom fan CFM rated at least 200.

Read Also: Top 3 Best Rated Quiet Bathroom Exhaust Fans (2019 Review)

Should I Consider a Secondary Bathroom Exhaust Fan?

A recent study by the US Department of Energy suggests a localized exhaust fan offers increased efficiency when it’s time to regulate air quality in a room [3]. This becomes essential when the bathroom is large and/or split into multiple sections.

Certain properties are designed in this manner making it important to invest in a second bathroom exhaust fan.

For example, if a bathroom has a separate shower area (with a door), it’s never a bad idea to set up an additional fan. This ensures the enclosed space is properly taken care of. Otherwise, air in that part of the bathroom will go untouched leading to unwanted odors and mold.

When measuring for a secondary space, it’s best to use the same calculation as before.

However, most secondary spaces such as enclosed shower areas require nothing more than a simple 50 CFM bathroom exhaust fan.

Remember, the primary exhaust fan is already regulating the rest of the bathroom.

If you don’t want to install a secondary fan, it’s highly recommended to leave the shower door open throughout the day. This allows the primary bathroom exhaust fan an opportunity to do its job. While it won’t be as effective as a secondary fan, there’s still a chance to see satisfactory changes in air quality.

Concluding Thoughts On Bathroom Fan Sizing

That’s all you need!

Proper bathroom fan sizing is an important investment and comes with several requirements. This is why it’s smart to take the measurements as early as possible before spending hard-earned money on a brand-new fan.

By using the formulas mentioned here, the fan will work like a charm.

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Arie Van Tuijl

Arie Van Tuijl

Arie is a licensed home inspector who owns a residential and commercial inspection company in the state of Virginia. He also does specialty testing such as radon gas, termites, air quality, and mold.

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