create a formula

If what you meant to ask was how to perform calculations on your form, we have several guides that might help.

How-to-Perform-Calculation-in-the-Form

How-to-Assign-Calculation-Value

Form-Calculation-Math-Function-Reference

In case we misunderstood your question, just get back to us with a bit more info and a clearer description of what you're trying to do so we can better assist you.