DBA_FREE_SPACE - UNDO tablespace

[tamilselvan]

What's your oracle release?

Try this:

col allocated for 999,999.999
col free      for 999,999.999
col used      for 999,999.999

select
    ( select sum(bytes)/1024/1024 from dba_data_files
       where tablespace_name like 'UND%' )  allocated,
    ( select sum(bytes)/1024/1024 from dba_free_space
       where tablespace_name like 'UND%')  free,
    ( select sum(bytes)/1024/1024 from dba_undo_extents
       where tablespace_name like 'UND%') USed
from dual
/