Fastest way to calculate file size, outputs 'B', 'KB', 'MB&

So far, I found that it may be the fastest calculation method
Is there a faster one?

date_default_timezone_set('UTC');

function format_bytes($size)
{
 $arr = array(' B', ' KB', ' MB', ' GB', ' TB ');
 for ($f = 0; $size >= 1024 && $f < 4; $f++)
 {
 $size /= 1024;
 }
 return round($size, 2).$arr [$f];
}
echo format_bytes('123456789');
?>