Disk Space used by a Table in Oracle

 How to find the Disk Space used by a table in Oracle, Here is the query

SELECT   segment_name table_name,
         SUM(BYTES) /(1024 * 1024) table_size_meg
FROM     user_extents
WHERE    segment_type = 'TABLE' AND
         segment_name = 'MY_TABLE'
GROUP BY segment_name