Calculate UNDO_RETENTION for given UNDO Tabespace

UNDO_RETENTION is a parameter in the init.ora initialization parameters file that specifies the time period in seconds for which a system retains undo data for committed transactions. The flashback query can go upto the point of time specified as a value in the UNDO_RETENTION parameter.

Optimal Undo Retention =
           Actual Undo Size / (DB_BLOCK_SIZE × UNDO_BLOCK_REP_ESC)

Actual Undo Size

SELECT SUM(a.bytes) "UNDO_SIZE"
FROM v$datafile a,
       v$tablespace b,
       dba_tablespaces c
WHERE c.contents = 'UNDO'
   AND c.status = 'ONLINE'
   AND b.name = c.tablespace_name
   AND a.ts# = b.ts#;

Undo Blocks per Second

SELECT MAX(undoblks/((end_time-begin_time)*3600*24)) "UNDO_BLOCK_PER_SEC"
FROM v$undostat;

DB Block Size

SELECT TO_NUMBER(value) "DB_BLOCK_SIZE [KByte]"
FROM v$parameter
WHERE name = 'db_block_size';

Optimal Undo Retention Calculation

Formula:
Optimal Undo Retention = 
           Actual Undo Size / (DB_BLOCK_SIZE × UNDO_BLOCK_REP_ESC)

Using Inline Views, you can do all calculation in one query

SQL Code:
SELECT d.undo_size/(1024*1024) "ACTUAL UNDO SIZE [MByte]",
    SUBSTR(e.value,1,25)    "UNDO RETENTION [Sec]",
    ROUND((d.undo_size / (to_number(f.value) *
    g.undo_block_per_sec)))"OPTIMAL UNDO RETENTION [Sec]"
  FROM (
       SELECT SUM(a.bytes) undo_size
          FROM v$datafile a,
               v$tablespace b,
               dba_tablespaces c
         WHERE c.contents = 'UNDO'
           AND c.status = 'ONLINE'
           AND b.name = c.tablespace_name
           AND a.ts# = b.ts#
       ) d,
       v$parameter e,
       v$parameter f,
       (
       SELECT MAX(undoblks/((end_time-begin_time)*3600*24))undo_block_per_sec
       FROM v$undostat
       ) g
WHERE e.name = 'undo_retention'
  AND f.name = 'db_block_size'

Calculate Needed UNDO Size for given Database Activity

If you are not limited by disk space, then it would be better to choose the UNDO_RETENTION time that is best for you (for FLASHBACK, etc.). Allocate the appropriate size to the UNDO tablespace according to the database activity:

Formula:
Undo Size = Optimal Undo Retention × DB_BLOCK_SIZE × UNDO_BLOCK_REP_ESC
Using Inline Views, you can do all calculation in one query

SQL Code:
SELECT d.undo_size/(1024*1024) "ACTUAL UNDO SIZE [MByte]",
       SUBSTR(e.value,1,25) "UNDO RETENTION [Sec]",
       (TO_NUMBER(e.value) * TO_NUMBER(f.value) *
       g.undo_block_per_sec) / (1024*1024)
      "NEEDED UNDO SIZE [MByte]"
  FROM (
       SELECT SUM(a.bytes) undo_size
         FROM v$datafile a,
              v$tablespace b,
              dba_tablespaces c
        WHERE c.contents = 'UNDO'
          AND c.status = 'ONLINE'
          AND b.name = c.tablespace_name
          AND a.ts# = b.ts#
       ) d,
      v$parameter e,
      v$parameter f,
       (
       SELECT MAX(undoblks/((end_time-begin_time)*3600*24))
         undo_block_per_sec
         FROM v$undostat
       ) g
 WHERE e.name = 'undo_retention'
  AND f.name = 'db_block_size'