wayzer,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script src="http://code.jquery.com/jquery-1.9.1.js">
</script>
<style type="text/css">
body{
height: 1000px;
}
</style>
<script>
$(function ()
{
alert ($(document).height()- $(window).height())
}
);
</script>
</head>
<body>
</body>
</html>