# Bathroom Fan Sizing Guide: Code, CFM Chart & Calculation

Ventilation is essential in the bathroom. An adequately sized bathroom fan is critical to the air quality in the bathroom. So, how do you calculate the CFM of a bathroom fan?

To calculate the bathroom fan CFM, take the square footage of the bathroom and multiply it by one. For greater bathroom ventilation, multiply the square footage of the bathroom by 2 to get the bathroom fan CFM.

The CFM required in a bathroom is depending on several factors. Understanding how to calculate the CFM required for a bathroom allows you to size the bathroom fan based on your situation.

## How Much CFM is Needed for a Bathroom?

Bathroom fan CFM is referring to the airflow or capacity of the bathroom exhaust fan. CFM stands for cubic feet per minute. In other words, it shows how much exhaust airflow is needed to adequately ventilate a bathroom.

According to ASHRAE standard 62.1-2010, the exhaust CFM for a private toilet/bathroom is 50 CFM. At 8 air changer rate per hour, an exhaust fan with 50 CFM is adequate for a 50 sqft bathroom with 7.5 feet of ceiling height.

If the same bathroom has a ceiling height of 10 feet, the resulting air change rate per hour is 6.

So, generally, the minimum air change rate per hour required for bathroom is 6 assuming that the bathroom is occupied by one person at a time. If the bathroom continuously occupied by more than two persons, an air change rate per hour of 10 is recommended. For bathrooms that need to remove hot shower steam quickly, an air change rate per hour of 12 is recommended.

So, the minimum CFM required for a bathroom is equivalent to the total square footage of the bathroom. For instance, a 100 sqft bathroom requires an exhaust airflow of 100 cfm.

### Bathroom with Air Conditioning

In some houses, the bathroom is equipped with air conditioning. Often, a supply air diffuser can be found in the bathroom to provide some heating and cooling.

When cold or hot air is supplied to the bathroom, it creates a positive pressure in the bathroom. As a result, the air within the bathroom tends to flow into the adjacent room through the bathroom door gaps.

For example, if 50 cfm of conditioned air is supplied to the bathroom, 50 cfm of airflow is constantly flowing from the bathroom into the adjacent room.

But, the bathroom air is categorized as Class 2 by ASHRAE. Generally, Class 2 air has moderate contaminant concentration and mildly offensive odor but not necessarily harmful. On top of that, Class 2 air is inappropriate for transfer.

In other words, the bathroom air should not be flowing into the adjacent room and thus, it should always be in a state of negative pressure.

Therefore, if a 100 sqft bathroom has a supply air diffuser that is constantly supplying 50 cfm of conditioned air, the bathroom fan should have an exhaust airflow of 150 cfm; 50 cfm to balance out the positive pressure and 100 cfm to provide adequate ventilation.

Regardless, the bathroom should not have a return air grille as it violates the ASHRAE recommendation of inappropriate Class 2 air transfer. Depending on how smelly the bathroom is, the air conditioning system could potentially spread the smell to every corner of the house.

### Bathroom Fan CFM Chart

Bathroom fan CFM charts can be categorized into low, medium and high based on the required air change rate per hour and a fixed bathroom ceiling height.

Based on common bathroom sizes and floor areas, the following CFM charts provide the recommended bathroom fan CFM for adequate exhaust.

Below is the bathroom fan CFM chart based on 6 ACPH and 10 ft ceiling height:

At 6 ACPH, one bathroom square foot is equivalent to one CFM given that the ceiling height is 10 feet.

Below is the bathroom fan CFM chart based on 10 ACPH and 10 ft ceiling height:

At 10 ACPH, one bathroom square foot is equivalent to 1.67 CFM given that the ceiling height is 10 feet. Exhaust fans in the bathroom of office buildings are often sized based on 10 ACPH.

Below is the bathroom fan CFM chart based on 12 ACPH and 10 ft ceiling height:

At 12 ACPH, double the bathroom square foot to get the CFM. For bathrooms that have a hot water bathtub where steam is a concern, an air change rate per hour as high as 12 can be used to improve the ventilation rate.

## How to Calculate Bathroom Fan CFM?

The rule of thumb for bathroom fan capacity is one CFM per square foot. For example, a 100 sqft bathroom needs a 100 cfm bathroom fan.

Assuming that the bathroom has a ceiling height of 10 feet, the bathroom air volume is therefore 1000 cubic feet. Thus, the air change rate per hour in the bathroom can be calculated as follow:

Air Change Rate, ACPH = (60 x CFM) ÷ Air Volume
Air Change Rate, ACPH = (60 x 100) ÷ 1000
Air Change Rate, ACPH = 6

Therefore, the rule of thumb for bathroom fan capacity of one CFM per square foot will lead to 6 air change per hour.

However, the rule of thumb also pointed to an air change rate per hour of 8. Why is that?

If we reverse the air change rate per hour calculation, we can find out the assumed ceiling height in the 100 sqft bathroom as follow:

Air Change Rate, ACPH = (60 x CFM) ÷ Air Volume
8 = (60 x 100) ÷ (100 x Ceiling Height)
800 x Ceiling Height = 6000
Ceiling Height = 7.5 feet

Therefore, the one CFM per square foot rule of thumb for a bathroom fan assumed that the bathroom ceiling height is 7.5 feet.

In other words, if your bathroom ceiling height is lower than 7.5 feet, you’ll need less CFM. Vice versa, if your bathroom ceiling height is higher than 7.5 feet, you’ll need more CFM.

## Is Higher CFM Better for Bathroom Fan?

The ventilation rate of a bathroom is greater when the bathroom fan has a higher CFM. However, the higher the CFM, the greater the noise. Hence, higher CFM doesn’t necessarily mean better for bathroom fans.

Furthermore, higher bathroom fan CFM induces a greater negative pressure in the bathroom. If the adjacent room has air conditioning, energy will be wasted through the bathroom exhaust.

In addition, the net exhaust CFM in the bathroom at a degree is depending on the makeup air through the bathroom door gaps. If higher CFM is required, a separate fresh air vent is recommended.

The bathroom exhaust effectiveness is depending on the amount of makeup air replenished. If 150 cfm of airflow is exhausting out of the bathroom, 150 cfm of makeup air must replenish the bathroom from the adjacent room.

However, a typical bathroom door with a 1/2″ gap at the bottom may only allow approximately 50-100 cfm of airflow through. In this case, a 150 cfm bathroom fan may not actually be able to deliver 150 cfm of exhaust airflow.

Therefore, a dedicated fresh air vent is needed to allow more makeup air to flow into the bathroom as the bathroom fan is exhausting air out of the bathroom.

So, if the bathroom fan is more than 150 cfm given that the bathroom door gap is around 1/2 inch, a fresh air vent is recommended.

But, if the bathroom door gap is greater, a fresh air vent may not be needed. Vice versa, if the bathroom door has some sort of rubber seal, a fresh air vent is most likely needed.

To size a fresh air vent, see my post Return Air Grille Sizing Guide: CFM Chart & Calculation.

## What Happen If You Oversize a Bathroom Exhaust Fan?

Some people worry that mold will grow in their bathroom if they don’t provide sufficient ventilation in the bathroom. So, they want to stretch the capacity of their bathroom exhaust fan.

Generally, it is fine to put a larger exhaust fan in the bathroom. When you oversize a bathroom exhaust fan, the most noticeable consequence is the noise level. Large bathroom exhaust fans tend to be noisier. However, you can use two smaller bathroom exhaust fans to achieve the same result.

It is almost impossible to oversize a bathroom exhaust fan to a point that the negative pressure in the bathroom is so great that the bathroom door is unable to be opened.

Another consequence of an oversized bathroom exhaust fan is energy wastage as mentioned earlier. If the adjacent room has air conditioning, more conditioned air will be wasted through the bathroom exhaust.

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