What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
You can follow this guide on how to use conditional logic in your form: http://www.jotform.com/help/57-Smart-Forms-Using-Conditional-Logic.
If you wish to perform a calculation, just select "Update/calculate a field".
You'll need t set an "IF" statement on when the calculation will happen:
You can also perform a form calculation using the form calculation widget. Here's a guide to learn more about it: http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form
Let us know if that works for you.