calculate total on form

Hi there,
I have a form built from php that dynamically builds rows from mysql. each row has a total cost for the quantity of items purchased. I have done the javascript for that and it works fine. Now i wish for the form to add all the totals of each item to build a sub_total before VAT. Each line total input name has an index of “total_” and the name of the sub_total is s_total. Can someone give me some code that may work? thanks.

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