consegui hacerlo y ahora lo comparto
<script type="text/javascript" language="javascript">
function igv(){
base = parseFloat(document.getElementById('base').value);
iva=(base*19/100)
document.getElementById('iva').value = iva.toFixed(2);
total = (base*19/100)+base;
document.getElementById('total').value = total.toFixed(2);
}
</script>
