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.
I'm not sure if understood your question, this is our Form Calculation widget: http://widgets.jotform.com/widget/calculation.
Take a look on this link to get more details about this widget: http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form.
If it does not help, please provide us the link/name of your form as well as more details, we will be glad to help.