select status,
  round(sum_bytes / (1024*1024), 0) as MB,
  round((sum_bytes / undo_size) * 100, 0) as PERC
from
(
  select status, sum(bytes) sum_bytes
  from dba_undo_extents
  group by status
),
(
  select sum(a.bytes) undo_size
  from dba_tablespaces c
    join v$tablespace b on b.name = c.tablespace_name
    join v$datafile a on a.ts# = b.ts#
  where c.contents = 'UNDO'
    and c.status = 'ONLINE'
);

STATUS	      MB	  PERC
ACTIVE	     561	    0
EXPIRED	     12	            0
UNEXPIRED    124831        100

SQL> show parameter undo_retention;

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
undo_retention                       integer     900
SQL>

select status, count(*) Num_Extents, sum(blocks) Num_Blocks, round((sum(bytes)/1024/1024),2) MB
from dba_undo_extents group by status order by status

STATUS	     NUM_EXTENTS	NUM_BLOCKS	 MB
ACTIVE	         11	          64896	        507
EXPIRED	         72	          2608	       20,38
UNEXPIRED	8431	         15983184      124868,63

select tablespace_name, sum(bytes)/1024/1024/1024 GB 
from dba_data_files where tablespace_name='UNDOTBS1' group by tablespace_name;

TABLESPACE_NAME	GB
UNDOTBS1	122,46875

Oracle Database 11g Release 11.2.0.3.0 - 64bit Production

SQL> select sum(T.USED_UBLK) from v$transaction t;

SUM(T.USED_UBLK)
----------------
           18514

select sum(T.USED_UBLK) from gv_$transaction t;

select sum(T.USED_UBLK) from v$transaction t;

SQL> select min(t.TUNED_UNDORETENTION) from v$undostat t;

MIN(T.TUNED_UNDORETENTION)
--------------------------
                    141074