View Full Version : Throat length for rectangle 45deg elbows

12-06-2013, 09:23 AM
This may have been covered already, so if it does please let me know the thread or forward it to the correct thread. I am looking for a way to figure out the throat lengths for rectangle 45deg elbows with rec throats. For instance I have a 20x10 duct, and I need to drop in elevation 15". What will the throat lengths be and length of the finished offset be? It is easy on cad. But I am looking to do this in math and lay out on the floor of the construction site. Thanks

12-07-2013, 07:02 AM
Welcome to the forum.

Good question.

The throat would be determined not only by the drop (15") but by the distance (max length) of the drop. There is a link (formula) here in the library that ill find later when i get to my desk and share it here.

12-10-2013, 02:06 PM
Bud, did you find that info? Thanks. I'm surprised this is a subject that's so hard to get a answer on.

12-10-2013, 04:58 PM
Maybe this will help you? Let me know.


12-10-2013, 05:02 PM
FYI - for field work in a pinch.


12-11-2013, 07:13 AM
I appreciate your attempt to answer this. If you ever come across it, remembers to post it here.

12-11-2013, 07:34 PM
http://www.thesheetmetalshop.com/shared_images/offset/Offset_rd_pipe_template.jpg (http://www.thesheetmetalshop.com/shared_images/offset/Offset_rd_pipe_template.jpg)

OK, I'm just brainstorming here...Let me know if I'm out in left field too far:)

These are just lines? This round pipe can easily be understood as a rectangular piece of duct. Offset is Offset? Centerline radius is always good to understand and pretty simple once one understands it.

X is the distance from center to edge which would be the throat

For 45 degree elbows X = http://www.thesheetmetalshop.com/shared_images/offset/centerline_radius_sm.jpg R x .414

45 Elbow offsets L = ( O x 1.414) - 2 ( http://www.thesheetmetalshop.com/shared_images/offset/centerline_radius_sm.jpgR x .414)

So i'm thinking out loud here?

We want a 15 inch offset using 2 10" 45 degree elbows.

L (15 X 1.414) - 2 (10 X .414) <--This is X

21.21 - 8.28 = 12.93 (13")

L = 13"

Each throat would be 7 inches? (added to one side of elbow - X plus the 7 inches)

I'm just thinking out loud here? Is this making any sense?

12-11-2013, 07:36 PM
I'll check back tomorrow - this old man has to hit the sheets :) hope this helps.

12-12-2013, 04:17 PM
The cad program I draw with says the throats would be 8 9/16. I will keep looking at it.

12-12-2013, 04:30 PM
What cad program do you use?

12-13-2013, 04:48 AM
Your correct. My bad, in not explaining this right.:o

Your throat is a fixed length if you stick with a true centerline radius of 1 to 1 in this case. The 13 inches is the distance between the two offsets as in the formula explains.

The fixed throat (see drawing 1 to 2) will be about 2 1/16 inch plus half of the 13 is 6 1/2" will be 8 9/16.

You have to remember the allowance for the s-slips and drives.

Hope this helps. :)


You can easily take this information and create a chart or a cheat sheet.
Maybe I can do this while watching the Packers - it would be more interesting that's for sure. :10900:

12-13-2013, 10:11 AM
Autocad 2013 MEP and several add ons like intelicad, east coast cad and practicad. I can almost solve it but it requires that I use the SINE COSINE AND TANGENT commands on a calculator but I only know a little bit about how to use them.

12-13-2013, 10:13 AM
Yeah I will print it out for sure.

07-14-2014, 11:11 PM
Here would be your solution assuming you wanted to make the elbows 45degrees if you want it for any angle elbows the calculation is a little bit more intensive but not hard by any means.

07-14-2014, 11:14 PM
This is also assuming you are wanting to bolt the elbows back to back to make your offset. If your going to do it this way just order the offset in one fitting, however keep in mind that if the offset is greater then one forth of the length of the offset it will not meet smacna requirements. In that case you would need to order/build a equal offset that doesn't choke down the duct size.