Format file size (C#)

This post is not spectacular, but might be useful for some of you. I wrote a simple function to format the file size like it is in the file explorer (21 KB, 1,23 MB etc). I needed this for showing the file size in a tooltip for a SharePoint treeview.

private string FileSizeFormat(long size)
{
 const int KB = 1024;
 const int MB = 1048576;
 const int GB = 1073741824;

 if (size < KB)
 {
  return string.Format("{0} bytes", size); 
 }
 else if (size < MB)
 {
  return string.Format("{0} KB", (size / KB)); 
 }
 else if (size < GB)
 {
  return string.Format("{0:0.00} MB", (size / MB));
 }
 else
 {
  return string.Format("{0:0.00} GB", (size / GB)); 
 }
}

Just enter the file size (which usually is a long) into this function, you get the appropriate format back.
Edit: Made the code a bit easier (thanks Frank!)