Need Help with Formula

Hi, I am trying to create a bottom of duct tag for Revit Fabrication duct. So far I have been successful in getting the tag to work by using the offset value which gives me the center of the duct. I need help getting Dynamo to subtract the Main Primary Depth from the Offset to get the bottom value. Attached are shots of the progress I made. Can you help me figure this out?

Thank you,

JakeRevit shot1 Revit shot2 Dynamo shot1

If you get the correct values out you should be able to do the following within a minus node, their is also a node to convert a string to number which i have also indicated in the below.

I have also noticed that the code block you have named the code block incorrectly, you have spaces within your text.

EG Offset duct should be Offset_duct and this will get rid of the red code block error.


Hi Jake,

There is a node from “Spring nodes” Package called “Feet2Fraction” that will give you exact values as shown on Revit. I suggest before you proceed with your formula you better convert your units in Dynamo and then assign your values to “Bottom of Duct” for accurate result. Below screenshot is an example of duct.

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And i also i noticed your duct tag units (40"X18’) not matching with the values of Overall Size/Duct Depth (40"X18").


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Thank you both of you. I got it to work and fixed some other things also.


I’m having similar problem. can you screenshot your map?


Hey Russell,

Sorry I didnt see your post until now. I got it to work 99%. After messing around with it for a bit I found that when i move the duct it doesn’t update the tag unless I run the element string to each of the set.parameters but it comes up with an error. If you have any ideas please share.



Could you tell us what the error actually indicates?

It seems to function fine other than the message that the “Run completed with warnings” pops up. The other issue i am having is the BOD call out is reading as a negative. I am assuming i have something out of order in the formula.

Dynamo neg

Hi Jake,

Do all the fabrication parts have a top of duct and bottom of duct parameter? It might be better to use the Categories selection node with Duct Curves as Kulkul uses above.

I think your formulas should be Offset+Main_Primary_Depth/2 and Offset-Main_Primary_Depth/2.

Try setting only one of the parameters (either Top or Bottom)

In the family, the other should mostly be programmed to adjust correspondingly.

I ended up using this: Main_Primary_Depth/2+Offest-Main_Primary_Depth and it got rid of the -.

Hey Jake,

I’m just getting started with Dynamo and found this thread quite useful. However, how did you get the ‘Bottom of Duct’ parameter into the Properties of the family. I’ve not figured out how to create a parameter for Fabrication Parts. Is it an SP? If so how do I get it to show up?

Thanks for any help.