codice:
<html>
<head>
<title>Documento senza titolo</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<script>
var contaLettere = function(inpt){
   var tmp = inpt.value.replace(/ /g, '');
   return tmp.length;
}

window.onload = function(){
   document.getElementById('teso').onkeyup = function(){
       var count = contaLettere(this);
       document.getElementById('numero').innerHTML = count;
   }
}
</script>
</head>
<body>
<input type="text" name="teso" id="teso"><div id="numero"></div>
</body>
</html>