This is a limitation of SOQL aggregate queries.  Sorry. 

The very ugly work around is to create a text field that gets updated with workflow rules every time your formula field is updated.  That text field will be available to aggregate…

Thank you for the clarification.

This reply was created from a merged topic originally titled Can’t Group by Salesforce formula field in Agg Model.. I don’t have the option to group an aggregate model using a salesforce formula field. Is that a limitation, or am I just doing something wrong?

Is there another workaround option, e.g., something that takes advantage of aggregation in charts?  It is impractical to create a workflow-updated field for every formula field one might want to group by (plus all existing records would need to be updated to trigger the workflow).  

For example, if I want to group by contacts’ age at the time they started services (a custom formula field, let’s call it AgeAtStart), I can create an aggregate model, group by contact, and include AgeAtStart as an aggregation (with min or max as the aggregate function).  I can then create a chart based on that model to show the number (and maybe percent?) of contacts in each age group by splitting on the minAgeAtStart aggregation, using contact as the data field, and a count aggregate function in the chart setup.  So that gets most of the way there.  But, I also need those chart results in a table.  Is that possible?

I know I can get these results in standard Salesforce reports, which do allow grouping by a formula field, but I am trying to include the age results within a Skuid page that is reporting on a number of areas from one page.

Use process builder to take the result of the formula and place it in a non formula field - you can then use that to group

I tried this but the process builder does not trigger when the formula field is updated.

Am I thinking of this the wrong way?

Other way round use the process builder to take result from formula field and put it in a plain number/ text field you can group on. Process builder will trigger on record update/ create, not on formula update. Alternatively, take a look at Andyin the cloud declarative roll up summaries if your formula sums other records.

I am running at a hot spot that I need to aggregate the bunch of formula field changes, can someone please suggest on this if you implemented this.