Script to find Database size in Oracle

select t.totaloc "TOTAL ALLOCATED(bytes)",
       (t.totaloc - f.totfree) "TOTAL USED",
       f.totfree "TOTAL FREE(bytes)",
       ((t.totaloc - f.totfree)/t.totaloc)*100 "% USED"
  from (select sum(bytes) totaloc from dba_data_files) t,
       (select sum(bytes) totfree from dba_free_space) f ;