excerpt-character-counter.php

function excerpt_count_js(){

if ('page' != get_post_type()) {

      echo '<script>jQuery(document).ready(function(){
jQuery("#postexcerpt .handlediv").after("<div style=\"position:absolute;top:12px;right:34px;color:#666;\"><small>Excerpt length: </small><span id=\"excerpt_counter\"></span><span style=\"font-weight:bold; padding-left:7px;\">/ 500</span><small><span style=\"font-weight:bold; padding-left:7px;\">character(s).</span></small></div>");
     jQuery("span#excerpt_counter").text(jQuery("#excerpt").val().length);
     jQuery("#excerpt").keyup( function() {
         if(jQuery(this).val().length > 500){
            jQuery(this).val(jQuery(this).val().substr(0, 500));
        }
     jQuery("span#excerpt_counter").text(jQuery("#excerpt").val().length);

   });
});</script>';
}
}
add_action( 'admin_head-post.php', 'excerpt_count_js');
add_action( 'admin_head-post-new.php', 'excerpt_count_js');