Tablespace- Disk usage check

Query to check free space in each tablespace

The following query will show how much free space is left in each tablespace. A space check would be the first step in diagnosing and rectifying errors like
'ORA-01654: unable to extend index scott.my_idx by 128 in tablespace '

SELECT df.tablespace_name TABLESPACE,
       df.total_space TOTAL_SPACE,
       fs.free_space FREE_SPACE, 
       df.total_space_mb TOTAL_SPACE_MB,
      (df.total_space_mb - fs.free_space_mb) USED_SPACE_MB,
       fs.free_space_mb FREE_SPACE_MB,
       ROUND(100 * (fs.free_space / df.total_space),2) PCT_FREE
FROM (SELECT tablespace_name, 
             SUM(bytes) TOTAL_SPACE,
             ROUND(SUM(bytes) / 1048576) TOTAL_SPACE_MB
      FROM   dba_data_files
      GROUP BY tablespace_name) df, 
     (SELECT tablespace_name, 
             SUM(bytes) FREE_SPACE,
             ROUND(SUM(bytes) / 1048576) FREE_SPACE_MB
      FROM   dba_free_space
      GROUP BY tablespace_name) fs
WHERE df.tablespace_name = fs.tablespace_name (+)
ORDER BY PCT_FREE asc;

If you find that a Tablespace is lacking in freeSpace, you might want to add extra space to it by issuing the following command.

ALTER TABLESPACE TABLESPACENAME
ADD DATAFILE '/slot01/oracle/mydbdata/tablespacenamexx.dbf' SIZE 500 M

With Excerpts from: http://www.psoug.org/reference/tablespaces.html