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 need to add "Form Calculation" widget in your form to do all the calculation with in your form.
To use drop down menus for calculation, you can take advantage of our "Update a form field value" condition feature to read the selected value of the drop down and the add the calculation based on that. The following guide should help you: https://www.jotform.com/help/268-How-to-Insert-Text-or-Mathematical-Calculation-into-a-field-using-a-Condition
It is also possible to add the calculation value of every drop down options which can be used for calculation in form calculation widget. Please check the screenshot below on how to add the "Calculation Values" for drop down options:
Hope this helps.
Do get back to us if you have any questions.