Saturday, 12 December 2015

How to Automate your Familes with Formulas

Following on from our Dynamo BIM blogs we are going to look at using formulas in Revit families. Clients and people on training course often ask how can I do this or that within a family. The only way they can achieve what they want it to do is to use formulas.
A lot of Revit users are familiar with making a typical family using parameters and constraints. What most people don’t realise is that you can add formulas to families so calculations can be  
 done within the family.

In this blog I am going to give a high level overview of formulas in families. You don't need to be a programmer. But the usual family rules apply. Plan your family beforehand and flex (test)  your family as you go. The idea with formulas is to get parameters driving other parameter and automating your family as much as possible.
Let’s take a very basic arithmetic formula inside a family. In the screen shot below you can see that the width of the family is always going to be half the height.
The width is controlled by a simple formula Width = Height divided by 2. This can be a huge time saving for the user but also reduces the possibility of errors. When your under pressure to get a model it can be very easy to forget to change a parameter. So the more you can automate the family the increase you will get in productivity.
Within the family you can do very simple to advanced formulas using the following formula types
Basic Arithmetic

This includes basic addition, subtraction, multiplication & division 


Log of 1000 to base 10 is 3. 10 x 10 x 10 = 1000. In a Revit family it is written as log (1000). 


Raising x to the power of Y   X = X ^ Y . In Revit this is written exp(x)

Square Root

You can use the formula sqrt(16) 


Revit supports trigonometry a sample is  a = b / tan(B) 

Time to dust off that old maths book 
Conditional Statements
Revit family formulas also support conditional statements. A conditional statement checks if a condition is true or false and takes appropriate depending on the test.   

If Statement

The if statement test to see if a condition is true or False


In the example above if Height is greater then 1800mm the depth is 1500mm.  If the Height is less the depth is 1700mm.  

And, OR statements can also be used in Conditional formulas giving you additional power to check a condition. 

IF ( AND (x = 100mm , y = 200mm), 80mm, 30mm ) 

IF ( OR ( A = 100mm , B = 300mm ) , 80mm , 30mm ) 

You can also nest your conditional statements that is have 1 conditional statement inside another 

IF ( Length < 350mm , 300mm , IF ( Length < 450mm , 400mm, IF ( Length < 550mm , 500mm, 800mm ) ) )

If you really want to create powerful advanced families get into using formulas in your families. It will take time to get use to Revit’s formula format but its well worth it. 

If you have any questions on anything in this blog or any other blog or any of our services please feel free to contact me at

Dan Deery is the managing director of Siscín. Dan has over 20 years in the construction industry and has been using BIM since the mid 90’s. He also worked as a programmer for a CAD software company. Siscín  specialise in BIM Consultancy. training & visualisation and animation for the Infrastructure, Engineering, Construction, Facilities and Architectural industries. Check out our Website

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