Database used Size and Free space

select round(sum(used.bytes) / 1024 / 1024/1024 ) || ' GB' "Database Size",
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;