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.
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You can indeed apply calculation to field entries by using a calculation widget:
You would then add your fields into the calculation:
Depending on what the final result needs to be, you may need some parenthesis in there to separate out the values. For example if you want width x height then that number divided by the rest, your equation would need to be (width field * height field) / (144 * 12 * quantity field).
Here is your equation setup in a clone of your form:
You can clone my copy to see the full setup if you'd like:
If you have any further questions, let us know and we will be happy to help.