[CODE]<script language="VBScript">
sub ss()
if form1.t1.value="" or not isnumeric(form1.t1.value) then form1.t1.value=0
if form1.t2.value="" or not isnumeric(form1.t2.value) then form1.t2.value=0
if form1.t3.value="" or not isnumeric(form1.t3.value) then form1.t3.value=0
if form1.t4.value="" or not isnumeric(form1.t4.value) then form1.t4.value=0
form1.t4.value=eval(form1.t1.value)+form1.t2.value+form1.t3.value
end sub
</script>
<form name="form1" method="post" action="">
 <input name="t1" type="text" id="t1" value="0" size="10" onKeyUp="ss()">+
 <input name="t2" type="text" id="t2" value="0" size="10" onKeyUp="ss()">+
 <input name="t3" type="text" id="t3" value="0" size="10" onKeyUp="ss()">=
 <input name="t4" type="text" id="t4" value="0" size="10" onKeyUp="ss()">
</form>
[/CODE]