Getting the size of a pre-defined folder and printing on screen the percentage and space available based on a value.

usage.php

<?php
class Helpers
{
  public static function dirSize($dir) {

          if(is_file($dir)) return array('size'=>filesize($dir),'howmany'=>0);

          if($dh = opendir($dir)) {
              $size=0;
              $n = 0;

              while(($file=readdir($dh))!==false) {
                  if($file=='.' || $file=='..') continue;

                  $n++;
                  $data = self::dirSize($dir.'/'.$file);
                  $size += $data['size'];
                  $n += $data['howmany'];
              }

              closedir($dh);
              return ['size' => $size, 'howmany' => $n];
          }

          return ['size' => 0, 'howmany' => 0];
      }

      public static function formatSize($fsizebyte) {
          if ($fsizebyte < 1024) {
              $fsize = $fsizebyte." bytes";
          }elseif (($fsizebyte >= 1024) && ($fsizebyte < 1048576)) {
              $fsize = round(($fsizebyte/1024), 2);
              $fsize = $fsize." KB";
          }elseif (($fsizebyte >= 1048576) && ($fsizebyte < 1073741824)) {
              $fsize = round(($fsizebyte/1048576), 2);
              $fsize = $fsize." MB";
          }elseif ($fsizebyte >= 1073741824) {
              $fsize = round(($fsizebyte/1073741824), 2);
              $fsize = $fsize." GB";
          };
          return $fsize;
      }

      public static function getUsage($formatado = false, $console = false)
      {

          $path = "C:\caminho";

          $max = 21474836480; //20gb em bytes
          $dirSize = self::dirSize($path);
          $percentual = number_format(($dirSize['size'] * 100) / (double)$max, 2);

          if (!$formatado) {
              return $percentual;
          } else {
              return [
                  'usado' => self::formatSize($dirSize['size']),
                  'percentual' => $percentual,
                  'disponivel' => self::formatSize($max)
              ];
          }
      }
}

$armazenamento = Helpers::getUsage(true);
?>

<div class="col-md-3 pull-right">
    <div class="col-md-12 well">
        <h5 class="title-armazenamento">Armazenamento utilizado: <span class="label <?php echo $labelUtilizado; ?>"><?php echo $armazenamento['usado'];?>
            (<?php echo $armazenamento['percentual'];?>%)</span> de
            <span class="label label-default"><?php echo $armazenamento['disponivel'];?></label> disponíveis</h5>
    </div>
</div>