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 do that using the form calculation widget, here's the guide for that: http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form.
You can also use the option "update a form field" under conditional logic to pass values to fields: http://www.jotform.com/help/57-Smart-Forms-Using-Conditional-Logic.
If you are creating an order form and want to pass your calculation to your payment total field, then this guide might help you: http://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field.
Do let us know if you need more help on this.