database size

set linesize 100
col database_size format a30
col used_space format a30
col free_space format a30
select
 sum(used.bytes) / 1024 / 1024 / 1024  || ' GB' Database_Size,
 sum(used.bytes) / 1024 / 1024 / 1024  - free.space / 1024 / 1024 / 1024 || ' GB' Used_space,
 free.space / 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) space
 from dba_free_space) free
 group by free.space;