Are Bathroom Fans Standard Size

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So it’s time to replace your bathroom exhaust fan. Before you rush to buy the replacement a question pops into your mind; Are they all the same size? They all look similar from the surface but are they really?

Bathroom fans are not standard size. The housing dimensions vary across different models. The duct size is standardized. On most bathroom fans there is a 4 or 6-inch duct. By measuring the existing hole in the ceiling and noting the duct position, it is often possible to swap the fan without major drywall work.

Let’s take a look at what measurements you should take to make sure the new bath fan fits like a glove. If you are especially thirsty for knowledge, find out the working principle behind the bathroom fan.

Related article: 6 Easy Steps To Choose The Right Bathroom Fan

Cutout Dimensions

A cutout dimension is the hole you have in your ceiling or wall where your existing fan is mounted.

This determines the external dimensions of the fan housing. The easiest way to find out yours is to remove the fan cover and measure the opening in the drywall/ tiles.

Guide on how to remove the fan cover

bathroom fan grille cover being drilled open

There are some common dimensions that are used, but they are not standardized like for example wall sockets.

If you don’t find an fan that is an exact fit for the existing hole, then it is wise to buy a fan that is slightly larger than the the hole. That way you can simply trim the drywall to fit the new larger unit.

Here Is a table of the most common cutout dimensions and an example for each.

Size (Height)Duct SizeFanView on AmazonLink to Manual
12 1/4 x 12 1/4 inch (11 3/4)8Broan L 300ViewManual
10.2 x 10.2 inch (5.6″)3-4″Panasonic FV-0811VFL5EViewManual
10 1/2 x 11 3/8 (4″)3-4″Nutone LPN80ViewManual
8 1/4 x 14 1/4 inch (5 3/4″)4″Broan 765H80LBViewManual
9 1/2 X 10 Inch (5 3/4″)4″Broan AE 110ViewManual
9 1/2 x 10 Inch (5 3/4″) 4″Broan AE 110LK FlexViewManual
8 x 8 1/4 Inch (5 3/4)4″Broan 678ViewManual
Bathroom fan sizes

Housing Height

Depending on the dimensions of your ceiling joists the height fan you can use will be limited.

2 x 6 and 2 x 8 joists are commonly used in ceilings. Most ceiling fans are made so they will fit in 2 x 6 construction.

It is best to remove the old fan before buying a replacement, as there can be other obstructions above that can limit

Related Article: 7 Reasons Why Bathroom Fan Is Not Removing Steam

If you don’t want to remove your existing fan before buying a replacement then get one with a similar cutout dimension and 4-inch height. It is very unlikely that there is less than 4 inches of space in your ceiling.

Here is a example of a low profile 4 inch high bathroom fan from Amazon.

Duct Size

It is possible to connect the fan to both larger and smaller ducts by using adapters. But it is not recommended to connect a fan to a smaller duct than what is on the fan. This will make the fan work harder and it will be significantly louder than in the specifications.

Here is a universal adapter from Amazon that can be used to convert between different duct sizes.

Flexible duct

The duct size is usually determined by the size of the bathroom. In small bathrooms, a 4-inch duct is more than enough.

However in larger master bathrooms a 6 or 8 inch duct is used.

Related Article: Are Bathroom Fans Necessary

Duct size is also depends on the total length and complexity of the duct-work. Generally, the longer and more complex the system the larger the duct must be.

However, you should be aware that international and local building codes limit the length of the the ductwork based on factors like fan size, duct diameter, and number of joins. I have compiled some helpful tables to ensure that your house remains up to code after installation of a bathroom ventilation system.

Air flow (CFM)

CFM determines how much air the fan will move, there is a good rule of thumb when determining the CFM. For every square foot, you need at least 1 CFM. For example a 12 by 12 bathroom (144 sq. ft) would need a 144 CFM fan.

It can be beneficial to buy a larger capacity fan and then slowing it down with a speed controller.

Then you have the possibility to clean the air quickly when necessary. For example, after cleaning, to clear out dangerous chemicals. During normal operation, you can leave it on a low setting and enjoy quiet ventilation.

Bathroom exhaust fan airflow CFM chart

Learn more about CFM

Sound Level

This is measured in Sones. Anything under 1 sone is considered quiet. Keep in mind if you buy a high-capacity fan you can reduce the sound level for normal usage by slowing down the fan with a speed controller.

You can find a Sone to Decibel calculator here

Here I have a list of 8 super quiet bathroom fans. They are truly the quietest on the market today and I will keep this list updated.

There are bathroom fans on the market that are rated at less than 1 Sone. Those will be near-silent and are best if you want to keep the fan running constantly.

Learn more about Sones

Other Things To Consider

Room Side Mounting

Some exhaust fans can be mounted from the room side and require no attic access. Those are obviously preferable. Even if you have attic access it is not likely that going there will be any fun.

Most likely there is insulation on top of the fan that has to be removed and some attics don’t even have walkways.


While replacing the fan might be easy enough, do not attempt to work on wiring yourself if you don’t know what you are doing. This can lead to injury fines or even trouble with your insurer in case of an accident.

Removal Of Existing Fan

Random electrical wirings

Even if you buy a fan that can be installed from room side. The existing fan could be one that is installed from the attic and it could be impossible to remove it without attic access.

Duct Angle And Position

The existing duct in the ceiling could be coming in from the wrong side, or at an funny angle. It can be difficult to reposition the duct since it is usually in-between insulation.

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