multiplying columns adding rows for a final total

I'm building an invoice tracking tool where there are single line text fields like this:
item# | Description | Qty | Price| Total
I need it to take what is entered into QTY and Price to create a total for each row and then at the end of the form calculate the total. Normally I'd use Javascript for this, but I was looking through the code base and not really sure where to add it.
Thanks.