With a bathroom exhaust fan, you have to install adequately sized ducting. The problem is that there is no universal standard bathroom exhaust fan duct size. We are going to look at all duct sizes for bathroom fans (you can check the all-encompassing 4-9 inches ducts fitting 50-250 CFM exhaust fans further on).
Namely, bathroom fans use round ducts with 4-inch, 5-inch, 6-inch, 7-inch, 8-inch, and 9-inch. 4-inch and 6-inch ones are the most common ones.
The size of bathroom exhaust fan ducts for venting depends on:
- CFMs (primary factor). Bathroom fan airflow (measured in CFMs). The bigger the bathroom exhaust fan you have (more CFMs), the larger should the duct diameter be to facilitate all those CFMs.
- Length of the duct and number of elbows in the ducts (secondary factor). If you have long ducts (over 10 feet), you will have to use a larger duct diameter. In more than 80% of cases, you don’t really have to worry about bathroom fan duct length.
Here are some most common examples (you can find duct diameter sizes for all CFM bathroom fans in the chart below):
- 80 CFM bathroom fan will require a 4-inch duct diameter.
- 110 CFM bathroom fan will require a 5-inch duct diameter.
Most of these recommended exhaust fan duct sizes stem from the 2018 International Residential Code (IRC) Chapter 15: Exhaust Systems and, specifically, “Table M1507.4 Prescriptive Exhaust Duct Sizing”.
We will look at what size ducts you need for different bathroom exhaust fan sizes (including 50 CFM, 80 CFM, 110 CFM, 150 CFM, 200 CFM, and 250 CFM fans). These diameters are listed in the all-encompassing chart you will find below.
First of all, however, it’s very insightful to see how the size of an exhaust fan (in CFMs) determines the duct diameter:
Duct Diameter For Bathroom Fans With Different CFMs
With bathroom exhaust fan duct sizing it is important to understand that:
- CFMs a duct can handle is not directly connected with duct diameter (there is no linear relationship here).
- CFMs a dut can handle is directly connected with the area (cross-section) of the duct (here we have a linear relationship).
We can calculate the cross-section of a duct with a certain diameter with this simple area equation:
Duct Cross-Section = (π × (D/2)2) / 2
As you can see, the airflow-carrying ability of the duct is proportional to the square of the diameter (D). When you calculate cross-sections for ducts with different diameters (using the above equation), you can be able to see a pattern:
- 4-inch duct has a cross-section of 6.28 square inches. This is a recommended duct size for a 50 CFM bathroom exhaust fan.
- 5-inch duct has a cross-section of 9.81 square inches. This is a recommended duct size for an 80 CFM bathroom exhaust fan.
- 6-inch duct has a cross-section of 14.13 square inches. This is a recommended duct size for a 110 CFM bathroom exhaust fan.
- 7-inch duct has a cross-section of 19.23 square inches. This is a recommended duct size for a 150 CFM bathroom exhaust fan.
- 8-inch duct has a cross-section of 25.12 square inches. This is a recommended duct size for a 200 CFM bathroom exhaust fan.
In all of these cases, a 1 square inch of the duct will allow for about 8 CFM airflow. This is a good rule of thumb when thinking about the bathroom exhaust fan duct size.
Based on this rule of thumb, here are recommended sizes for different bathroom fans:
Bathroom Exhaust Fan Duct Size Chart
|Exhaust Fan Size (CFMs):||Recommended Duct Diameter (Inches):|
|50 CFM Bathroom Fan||4 Inch Round Duct|
|70 CFM Bathroom Fan||5 Inch Round Duct|
|80 CFM Bathroom Fan||5 Inch Round Duct|
|110 CFM Bathroom Fan||6 Inch Round Duct|
|150 CFM Bathroom Fan||7 Inch Round Duct|
|200 CFM Bathroom Fan||8 Inch Round Duct|
|250 CFM Bathroom Fan||9 Inch Round Duct|
As you can see, these duct size to bathroom fan CFM pairings directly reflect the 8 CFM per square inch rule of thumb.
Now you can adequately choose the ducts for your bathroom fan. If you need some help with the choices, you should consult the list of the best bathroom exhaust fans here and the list of the quietest exhaust fans here.
Note: If you have a pre-existing 3-inch duct and a 4-inch bathroom fan exhaust, you might be tempted to use 4-inch to 3-inch exhaust reducer for bathroom fans. This might not be the best idea because the fan will have to work extra hard against higher duct pressure, thereby reducing its effectiveness.
With this rule of thumb in you, you will be able to calculate the bathroom exhaust fan duct size (diameter) yourself. If you have any questions, you can use the comment section below, give us some numbers, and we will try to help you out.
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