Note:I tried this only with the LEN function and i hope it works with other formula functions as well.

My requirement:

I wanted to calculate the length of a couple of fields of data type Text Area, and put the number into a new field.

As usual, i created a formula field and used the LEN function. When i tried to INSERT my Long Text Area fields, they were not available for selection. I later found out that you cannot use Long Text Area fields in formulas as of now. Hope that it becomes available soon, until then here is a workaround.

Let's say that i have an object named "MyCustomObject__c". I want to calculate the sum of the length of the Long Text Area fields Field1__c, Field2__c, Field3__c and put the value into SumofLenght__c

STEP 1: Create a new field on the object MyCustomObject__c named "SumofLength__c" of datatype "Number".

STEP 2: Create a new Workflow rule on the object "MyCustomObject__c"

Evaluation Criteria: Every time a record is created or edited
Rule Criteria: MyCustomOject Name: NOT equal to NULL


STEP 3:

Create a Workflow Field Update as below