Javascript: Multiply values in different in two input boxes

Well you can't do it with ID. An ID needs to be unique for each element in your DOM.

So instead you need to find the table by it id and then loop over each row. You then need to find the correct columns by class. This might help: http://stackoverflow.com/questions/3065342/how-do-i-iterate-through-table-rows-and-cells-in-javascript

Note: It's much easier to do this kind of stuff with jQuery or some other library, but that's up to you.

@bobbybouwmann I now have

<script type="text/javascript">
    var table = document.getElementById("assessTable");
    for (var i = 0, row; row = table.rows[i]; i++) {
        for (var j = 0, col; col = row.cells[j]; j++) {
            function calculate() {
                var quantity = document.getElementById('qty').value;
                var price = document.getElementById('prc').value;
                var total = document.getElementById('ttl');
                var result = quantity * price;
                total.value = result;
            }
        }
    }
    
</script>

but it isn't working

Did you read what I said? Your elements can't have the same ID, you now give them all the same ID... You need to select the element with a certain class or anything else, but not IDs.

function calculate(key) {
    var perUnitRate = document.getElementById('perUnitRate'+key).value;
    var totalUnite = document.getElementById('totalUnite'+key).value;
    var result = document.getElementById('result'+key);
    var myResult = perUnitRate * totalUnite;
    result.value = myResult;
}