script to find out oracle database size?

col "Database Size" format a20
col "Free space" format a20
col "Used space" format a20
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
 /