Mamun's DBA Blog: How To Check Undo Tablespace Size and Free Size in Oracle

First Connect through SQLPlus then,

SQL> select a.tablespace_name, SIZEMB, USAGEMB, (SIZEMB - USAGEMB) FREEMB
     from (select sum(bytes) / 1024 / 1024 SIZEMB, b.tablespace_name from dba_data_files a,
     dba_tablespaces b where a.tablespace_name = b.tablespace_name and b.contents = 'UNDO'
     group by b.tablespace_name) a, (select c.tablespace_name, sum(bytes) / 1024 / 1024 USAGEMB
     from DBA_UNDO_EXTENTS c where status <> 'EXPIRED'group by c.tablespace_name) b
     where a.tablespace_name = b.tablespace_name;

TABLESPACE_NAME     SIZEMB    USAGEMB        FREEMB
--------------- ---------- ---------- -------------------------------------------------
UNDOTBS1                        15360         50.6875           15309.3125
UNDOTBS2                        15360         40.5625           15319.4375