Thumb rule for sizing UNDO Tablespace Size

Thumb rule for sizing UNDO Tablespace Size

Sizing an UNDO tablespace requires three pieces of data.
(UR) UNDO_RETENTION in seconds
(UPS) Number of undo data blocks generated per second
(DBS) Overhead varies based on extent and file size (db_block_size)

The undo space needed is calculated as:
UndoSpace = UR * (UPS * DBS)

This query would give you the required minimum size in MB:

SELECT (UR * (UPS * DBS)/1024/1024) AS "Bytes" FROM (SELECT value AS UR FROM v$parameter WHERE name = 'undo_retention'),
(SELECT undoblks/((end_time-begin_time)*86400) AS UPS
FROM v$undostat
WHERE undoblks = (SELECT MAX(undoblks) FROM v$undostat)),
(SELECT block_size AS DBS
FROM dba_tablespaces
WHERE tablespace_name = (SELECT UPPER(value) FROM v$parameter WHERE name = 'undo_tablespace'));

Thumb rule from past experience: OEM 12c Cloud control suggests you set the size to 10 times of this value. I have tested for multiple clients it works well in 5 times in most of the cases, need observations afterwards.

Now, suppose your current undo_retention is 900 and you are increasing that to 9000. The required UNDO tablespace size would be(in MB):
Result_Of_Above_Query*5*10
5 is required as per thumb rule.
10 is required as you are increasing the UNDO_RETENTION value to 10 times.