Oracle buffer cache hit ratio

SELECT (1 - (v3.VALUE / (v1.VALUE + v2.VALUE))) * 100
v$sysstat v1,
v$sysstat v2,
v$sysstat v3
WHERE v1.NAME = 'db block gets'
AND v2.NAME = 'consistent gets'
AND v3.NAME = 'physical reads';

After get the buffer cache hit ratio, It must be over 95%.
To increase this value you have to increase the
DB_CACHE_SIZE