change
onkeypress="return validatenumerics(event)"
to
onkeyup="return validatenumerics(this)"
script:
<script>
var kgRate = 50;
function validatenumerics(e)
{
e.parentElement.nextElementSibling.children[0].innerText = (e.value * kgRate || 0);
}
</script>
P.S. Very straightforward way...