How do I calculate the table space size in oracle

SELECT   /* + RULE */
         df.tablespace_name "Tablespace",
         df.bytes / (1024 * 1024 * 1024) "Size(GB)",
         SUM (fs.bytes) / (1024 * 1024 * 1024) "Free(GB)",
         NVL (
            ROUND (
               SUM (fs.bytes) * 100 / df.bytes
            ),
            1
         ) "%Free",
         ROUND (
            (  df.bytes
             - SUM (fs.bytes)
            ) * 100 / df.bytes
         ) "%Used"
    FROM dba_free_space fs,
         (SELECT   tablespace_name, SUM (bytes) bytes
              FROM dba_data_files
             WHERE tablespace_name LIKE 'ONTD%'
          GROUP BY tablespace_name) df
   WHERE fs.tablespace_name(+) = df.tablespace_name
GROUP BY df.tablespace_name, df.bytes
ORDER BY 3 DESC