Query to see the Reserved, Used and Free space of your database

SELECT r.tablespace_name, reserved_space "RESERVED_SPACE(MB)",
reserved_space - free_space "USED_SPACE(MB)",
free_space "FREE_SPACE(MB)"
FROM (SELECT tablespace_name, SUM (BYTES)
/ (1024 * 1024) reserved_space
FROM dba_data_files
GROUP BY tablespace_name) r,
(SELECT tablespace_name, SUM (BYTES) / (1024 * 1024) free_space
FROM dba_free_space
GROUP BY tablespace_name) f
WHERE r.tablespace_name = f.tablespace_name
ORDER BY r.tablespace_name;