diff --git a/admin/include/functions.php b/admin/include/functions.php
index f587c3028..fac387eed 100644
--- a/admin/include/functions.php
+++ b/admin/include/functions.php
@@ -637,13 +637,17 @@ function get_fs_directory_size($path)
   $bytestotal = 0;
   $path = realpath($path);
 
-  if ($path !== false && $path != '' && file_exists($path))
+  if (!function_exists('exec'))
   {
-    foreach (new RecursiveIteratorIterator(new RecursiveDirectoryIterator($path, FilesystemIterator::SKIP_DOTS)) as $object)
-    {
-      $bytestotal += $object->getSize();
-    }
+    return false;
   }
+
+  @exec('du -sk '.$path, $returnarray);
+  if (is_array($returnarray) and !empty($returnarray[0]) and preg_match('/^(\d+)\s/', $returnarray[0], $matches))
+  {
+    $bytestotal = $matches[1] * 1024;
+  }
+
   return $bytestotal;
 }
 
diff --git a/admin/intro.php b/admin/intro.php
index 1e587d651..904e72626 100644
--- a/admin/intro.php
+++ b/admin/intro.php
@@ -404,8 +404,8 @@ if (isset($result[0]['SUM(filesize)']))
   $data_storage['Formats'] = $result[0]['SUM(filesize)'];
 }
 
-// PHP 5.5.19 is the oldest version get_fs_directory_size was tested on
-if (version_compare(PHP_VERSION, '5.5.19') >= 0)
+// Windows can't execute get_fs_directory_size correctly
+if (strtoupper(substr(PHP_OS, 0, 3)) !== 'WIN')
 {
   if (!isset($_SESSION['cachedir_info']) or $_SESSION['cachedir_info']['calculated_on'] < strtotime('5 minutes ago'))
   {