I am on 10.2.0.4 and DB size is 150gb. Each day I have about 20 redo log switches each of 1GB. I have my rman backup on tape/separate location. How do I calculate the FRA space for DB flash for 24 hours? This is just to flash back the DB to 24 hours if any logical corruption/garunteed retension.
Will you have both archivelog and flashback logs in the FRA ?
Really it all depends on your standards, budget, monitoring, tape drives, maintenance scripts etc.
How often do you backup the database ? How often do you backup archivelogs ? If you backup archives every 24 hours, then really you want to keep minimum 48 hours and preferably 72 hours on disk to allow for failed backups, bad tapes etc.
So ... rough estimate of 60GB for 3 days archivelog + 20GB for 24 hours flashback logs = 80GB.
You can reduce this by increasing archivelog backup frequency and reducing archivelog retention (backup archivelog all delete input).
Flashback log volume depends on what kind of transaction you are doing. Insert ? Update ? Delete ?
From experience flashback volume is generally less than archivelog, so I just use the archivelog volume as a safe estimate.
You may find some useful information here :-