Just try the code below may be it helps.
<html>
<head>
<script language="javascript">
function convertor(){
var curDpi = document.getElementById("curDpi");
var curSens = document.getElementById("curSens");
var newDpi = document.getElementById("newDpi");
var convert = parseInt(curDpi.value) * parseInt(curSens.value) / parseInt(newDpi.value);
document.getElementById("newSens").innerHTML = convert;
}
</script>
</head>
<body>
<form>
<p>Current DPI</p>
<input id="curDpi">
<p>Current Sensitivity</p>
<input id="curSens">
<p>New DPI</p>
<input id="newDpi">
<button id="convert" onclick="return convertor()">Convert</button>
<div id="newSens"></div>
</form>
</body>
</html>
Hope I understood your problem in a correct manner.