codice:
<!-- includo jQuery per sfruttarne il motore di selezione-->
<script language="javascript" type="text/javascript" src="https://ajax.googleapis.com/ajax/lib...n.js"></script>

<script type="text/javascript">
$(function(){
    $('.factor').keyup(function(){
   var t=$(this).parents('tr')
        var factors=$('.factor',t);
    $('input[name^="imp_"]',t).val(Number(factors.eq(0).val())*Number(factors.eq(1).val()));
    });
});
</script>

<!-- nota l'aggiunta della classe "factor" nell'HTML-->
<table>
   <td>Intervento</td>
   <td>Fornitore</td>
   <td>Q.ta</td>
   <td>Unitario</td>
     <td>Totale</td>
   </tr>
 <tr>
   </tr>
 <tr>
   <td><input name="itr1" class="uppercase" id="itr1" size="40"></td>
   <td><input name="for1" class="uppercase" id="for1" size="15"></td>
   <td><input name="Q.ta" class="uppercase factor" id="q1" size="2" value="0"></td>
   <td><input name="Imp.Unitario" class="uppercase factor" id="iu1" size="2" value="0.00"></td>
   <td><input name="imp_1" id="imp_1" size="4" value="0.00"></td>
   </tr>
 <tr>
   <td><input name="itr2" class="uppercase" id="itr2" size="40"></td>
   <td><input name="for2" class="uppercase" id="for2" size="15"></td>
   <td><input name="Q.ta" class="uppercase factor" id="q2" size="2" value="0"></td>
   <td><input name="Imp.Unitario" class="uppercase factor" id="iu2" size="2" value="0.00"></td>
   <td><input name="imp_2" id="imp_2" size="4" value="0.00"></td>
   </tr>
</table>