Determine used and free space in a tablespace

SELECT t.tablespace,
       t.totalspace AS " Totalspace(MB)",
       ROUND ( (t.totalspace - fs.freespace), 2) AS "Used Space(MB)",
       fs.freespace AS "Freespace(MB)",
       ROUND ( ( (t.totalspace - fs.freespace) / t.totalspace) * 100, 2)
           AS "% Used",
       ROUND ( (fs.freespace / t.totalspace) * 100, 2) AS "% Free"
  FROM (SELECT ROUND (SUM (d.bytes) / (1024 * 1024)) AS totalspace,
               d.tablespace_name tablespace
          FROM dba_data_files d
        GROUP BY d.tablespace_name) t,
       (SELECT ROUND (SUM (f.bytes) / (1024 * 1024)) AS freespace,
               f.tablespace_name tablespace
          FROM dba_free_space f
        GROUP BY f.tablespace_name) fs
 WHERE t.tablespace = fs.tablespace
ORDER BY t.tablespace;