Hello Ravi,
Good Day !!!
We may have n number of rows in Gridview we should mention the row index t find the text box value
Modify your script like this it will work
<script type="text/javascript">
function startCalc() {
var grid = document.getElementById("<%= gv1.ClientID%>");
for (var i = 0; i < grid.rows.length - 1; i++) {
var rate = $("input[id*=txtrate]") ;
var qty = $("input[id*=txtqty]") ;
var totalamount = rate[i].value * qty[i].value;
$("input[id*=lblamt]") = totalamount[i] .value;
}
}
</script>