How to find database size oracle

select round(sum(used.bytes) / 1024 / 1024 / 1024 ) ' GB' "Database Size"
, round(sum(used.bytes) / 1024 / 1024 / 1024 ) -
round(free.p / 1024 / 1024 / 1024) ' GB' "Used space"
, round(free.p / 1024 / 1024 / 1024) ' GB' "Free space"
from (select bytes
from v$datafile
union all
select bytes
from v$tempfile
union all
select bytes
from v$log) used
, (select sum(bytes) as p
from dba_free_space) free
group by free.p