JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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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.