Calculation on fields

Hi I have a form in suitecrm which has two fields, quantity and its price for several items. Now I want to show the customer his total amount in the field called Total. So basiclly I have a very simple mathematical calculation Quantity_Item1price+Quantity_Item2price+Quantity_Item3*price. Can any one pls help me how to achieve this without actually changing the view or saving the data in db?