<script>
function calcIt(textbox1, textbox2, feedbackId)
{
var mellem = parseInt(textbox1.value) + parseInt(textbox2.value);
document.getElementById(feedbackId).innerHTML = mellem * 0.5;

}

</script>


<div id="feedback" style="display: block;">&nbsp;</div>
<div>
<label for="calc1">Indtast Længde</label>
<input type="text" id="calc1" name="calc1" onKeyUp="calcIt(document.aspnetForm.calc1, document.aspnetForm.calc2, 'feedback');" value="0"> mm
</div>

<div>
<label for="calc2">Indtast Brede</label>
<input type="text" id="calc2" name="calc2" onKeyUp="calcIt(document.aspnetForm.calc1, document.aspnetForm.calc2, 'feedback');" value="0"> mm
</div>


<input type="button" onclick="calcIt(document.aspnetForm.calc1, document.aspnetForm.calc2, 'feedback');" id="udregn" value="udregn" name="udregn">