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Thanks for asking. It all depends. As my colleague jedcadorna explains in this thread, if decimals are to be included in the calculation, you will need to use number fields. However, if you want to use a different field type, there's always the option of embedding the form's full source into a web page and adding a script to the same page do the calculation, such as was done here.
Just elaborate on what you wish to accomplish and we'll do our best to help.