New Field When Ordering Product

[viszman]

Hi, is there a way to add field when ordering product. For example it should work this way:

I have cable, prize for each meter is 5€ and that addidional field will be used to customalize lenght and prize of cable, so when someone type 1,6 it this field prize will automaticly change to 8.

I think i need to modify some .tpl but don't know which one.