How to calculate the recommended online redo log size in Oracle

Rule of thumb: online redo logs should switch every ~20 minutes under peak load. This measures the average time between switches during your peak window over the last 3 days, then scales the current log size to hit that target. Prompts for peak-hour range in HH24:MI format (e.g. ''09:00'' and ''17:00'').

ACCEPT start_of_peak_hours CHAR PROMPT 'Peak hours start (HH24:MI, e.g. 09:00): '
ACCEPT end_of_peak_hours   CHAR PROMPT 'Peak hours end   (HH24:MI, e.g. 17:00): '

SELECT (SELECT ROUND(AVG(bytes) / 1024 / 1024, 2) FROM v$log) AS "Redo size (MB)",
       ROUND((20 / average_period) * (SELECT AVG(bytes) FROM v$log) / 1024 / 1024, 2)
         AS "Recommended Size (MB)"
FROM (
  SELECT AVG((next_time - first_time) * 24 * 60) AS average_period
  FROM   v$archived_log
  WHERE  first_time > SYSDATE - 3
  AND    TO_CHAR(first_time, 'HH24:MI')
         BETWEEN '&&start_of_peak_hours' AND '&&end_of_peak_hours'
);