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 might want use one of our payment tools to do the calculations. Please refer to this thread for additional info http://www.jotform.com/answers/92783-Can-I-do-calculations-in-my-forms-Also-can-I-get-values-formatted-at-money-115-00-.
Otherwise let us know the formula of the calculation that you want and we'll let you know if its feasible in the form builder, if not you have to download completely your source code and make some modification base on your calculation needs.