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Hi, Unfortunately we do not yet have a feature to create calculated fields in the forms. The best way to do this is using one of our Payment Tool Integrations which caculate this from a Qty option which I believe will best meet your need for this.
The only other way to do this is with the form's source code and getting your hands dirty with some custom programming to create a workaround for this. There is also a workaround using Dropdown boxes provided by one of my colleagues for this found here http://www.jotform.com/answers/202265-Calculate-a-grand-total-on-my-dropdown-form-fields if you'd like to see if this might interest you.