The main purpose of a bathroom fan duct is to conduct exhausted air from your bathroom to the outside. It is, therefore, important that the duct is correctly sized to create an optimal flow of air out the external vent.
Most bathroom fans use a 4″ or 6″ duct for installation. Read on to see what size is best suited for your bathroom needs.
4" and 6" ducts are both common sizes. Which size is best depends on the fan’s capacity rating, the duct material, and the length of the ductwork. For the most part, the connection point that the fan comes with is the best to use. Adapters should be avoided.
What Size is the Connection Point?
Most bathroom fans are made with either a 4″ or a 6″ connection point, which makes finding the right duct much easier as you would just need to choose the appropriate duct for your connection point.
You do get adaptors that allow you to fit a 6″ duct to a 4″ connection point and vice versa, but it is best to use the duct that will fit correctly on the connection point; otherwise, it impacts your airflow.
The fan capacity and the duct size are critically interconnected and manufacturers will use a connection point that fits the duct that will be the best for use with the fan and the amount of air it pulls into the duct at a time.
If you use a duct that is larger than recommended, then the airflow is negatively affected. If the fan pulls a certain volume of air into a duct that is too wide, the air would lose momentum. It would have a larger volume to fill, and so the pressure would decrease.
True, there would be less friction, but the concurrent loss of velocity pressure means the air will not move towards the terminal vent effectively (this is only true if the fan is underpowered).
If the duct was too narrow the friction or static pressure would be too great for the velocity pressure to overcome and the air also would not move through the ducts effectively. Using a narrower duct can also create a lot of noise.
Duct Size Depends on Bathroom Size and Fan Capacity
Some fans come with a single connection point but with the option to cut it in a way that turns the 4″ connection into a 6″ connection. So, what do you do in these cases?
In order to choose the correct duct size, you need to determine the correct Bathroom Fan Capacity (CFM). This involves using the size of your bathroom to help determine the fan capacity you need. From there, you will be able to find the appropriate diameter range for your duct. I have added a table below for your convenience.
| Bathroom Fan Capacity (CFM) | Appropriate Diameter Range | Recommended Diameter* |
| 50 | 3” to 4” | 4” |
| 80 | 4” to 5” | 4” |
| 100 | 4” to 6” | 6” |
| 125 | 5” to 6” | 6” |
| 150 | 6” to 7” | 7” |
| 200 | 7” to 8” | 7” |
| 250 | 7” to 8” | 8” |
| 300 | 8” to 9” | 8” |
The fan capacity will help determine the correct duct diameter to allow for efficient pressure to be released. Ducts that are 4” to 6” are suitable for most common bathroom fan capacity requirements and many fans will come with adapters for both diameter ranges, however, it is always important to look at your manufacturer’s instructions before installation.
Required Duct Length Affects Duct Diameter
If you need the fan to vent a certain distance, then you need to pick the duct size that will allow this length with the fan’s rating.
The tables below is adapted from Table M1504.2 of IRC, which specifies what duct diameter is needed in relation to the maximum ducting length required.
It is important to note that the type of ducting you use will impact the length of ducting allowed. There are definite advantages and disadvantages to the different types of bathroom fan ducting, so you will need to keep this in mind when deciding on what kind of duct to use.
It is also important to note that you will need to remove 15 feet from the allowable duct length for each elbow added. The IRC does not specifically limit the number of elbows you can use and as such, it is important to consult your local guidelines before installation.
Rigid Duct Length Sizing Table
Flexible Duct Length Sizing Table
Sources
https://codes.iccsafe.org/content/IRC2018/chapter-15-exhaust-systems