Pretty simple solution, with a few assumptions: 1. you load javascript functions 2. users have modern browsers

On the text box use this code: onkeyup="charcountupdate(this.value)"

After the text box, add this where you want to display the counter: <span id=charcount></span>

(can also use div instead of span)

And on the javascript file add these lines:

function charcountupdate(str){
var lng = str.length;
document.getElementById("charcount").innerHTML = lng + ' out of xxx characters';
}

see my example: http://codepen.io/borisplotkin/pen/RPwXeK

Hope this helps.