I need to run about 20 feet of duct to reach the gable, I don't want to go through the roof unless I absolutely need to, I live in Vermont and we can get a lot of snow build up on the roof, I also would prefer not to go through the eaves. Is this too long a run?


This will completely depend on the number of elbows, the size of the room being ventilated, and the power of the fan.

In general for a bathroom, you'll be looking to have an Air Change per Hour (ACH) of 8. To accomplish this, you'll have to select a fan based on the size of the bathroom, and the equivalent duct length.

Bath room size

The first thing you'll have to figure, is the size of the bathroom. For this, simply multiply Length (ft.) * Width (ft.)* Height (ft.). This will give you the size of the room in cubic feet.


Given a room that is 10 feet long, 8 feet wide, and 8 feet tall.

Volume = Length * Width * Height
Volume = 10 ft. * 8 ft. * 8ft.
Volume = 10 * 64
Volume = 640 cu. ft.

Equivalent Duct Length

To calculate the equivalent duct length, you'll have to consider not only the length of the duct. You'll also have to know the duct construction, and number of elbows. The basic rules that apply to both 3" and 4" duct, are as follows.

  1. Measure the length of straight duct.
  2. If the duct is flexible aluminium, multiply the length by 1.25.
  3. OR If the duct is flexible insulated, multiply by 1.5.
  4. For each elbow, add 15 feet.
  5. For each terminal (wall cap, roof jack), add 30 feet.


Given this duct:

enter image description here

There is 10' of straight smooth walled duct. Plus 2 elbows, and one terminal at the end.

EDL = 10' + (2 * 15') + (1 * 30')
EDL = 10' + 30' + 30'
EDL = 10' + 60'
EDL = 70'

Determine the proper sized fan

Once you've got all the information, it's time to select an appropriately sized fan. To do this, you can use a chart like this

Bathroom Fan Sizing Chart

Using this chart, you'll find that for a 640 cu. ft. bathroom and 70 ft. of duct (equivalent length), you'll need a 110 cfm fan to achive the recommended 8 air changes per hour.

  • See also this answer – Tester101 May 16 '14 at 12:39
  • I have two points I am hoping @Tester101 can clarify. Does placing the fan at the midpoint of the 5' length between the 2 elbows change the calculation? (vs. fan at bathroom or on exterior wall.) If there's a "T" in the duct, at the bathroom end, how does that change the calculation? – Fitter Man Dec 2 '18 at 18:15
  • @FitterMan the fan location makes no difference. As for adding a tee, I'm not sure what you mean, or why you would add a tee. – Tester101 Dec 8 '18 at 16:01
  • Regarding the tee, I want the fan to exhaust from 2 rooms (one's the main area of the bath with shower, the other has the toilet behind a separate door). So, there's one timer to activate the fan, but it has 2 openings, one in each area of the bathroom. Maybe "splitter" is the right word to use rather than tee. – Fitter Man Dec 9 '18 at 18:05

For a bath vent, no. If it was a dryer vent maybe so if it had more than 2 elbows.

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