<script type="text/javascript">
        function Rileva {
                if(typeof window.getComputedStyle!="function"){
                        window.getComputedStyle=function(elt,pseudoElt){
                        return (elt!=null && typeof
                        elt.currentStyle=="object")?elt.currentStyle:null;
                        }
                }
                var MyValue=getComputedStyle(document.getElementById("IdDelDiv"),'').width; //Restituisce ad esempio 300px
                var MyValueNum=partseInt(MyValue); //Restituisce ad esempio 300
        }
        //Fonte : http://www.criosoftware.it/default26.aspx
</script>