How to have Calculations on the Form

I have same need. Looking forward for that functionality in future editions.

I suggest reviewing the user guide also

http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form

You can also use Conditions and Calculations together

http://www.jotform.com/help/286-How-to-Populate-Field-with-a-Value-from-another-Field

If you can provide us which of your form you are working on, and more details of your requirement, we will be glad to assist you create the form.

Let us know if you need further assistance.