static string FormatBytes(long bytes)
{
	const int scale = 1024;
	string[] orders = new string[] { "GB", "MB", "KB", "Bytes" };
	long max = (long)Math.Pow(scale, orders.Length - 1);

	foreach (string order in orders)
	{
		if (bytes > max)
			return string.Format("{0:##.##} {1}", decimal.Divide(bytes, max), order);

		max /= scale;
	}
	return "0 Bytes";
}

Source: http://sharpertutorials.com/pretty-format-bytes-kb-mb-gb/