Sql command to check the size of database

select a.data_size+b.temp_size+c.redo_size "total_size" from ( select sum(bytes)/1024/1024/1024 data_size from dba_data_files ) a, ( select nvl(sum(bytes)/1024/1024/1024,0) temp_size from dba_temp_files ) b, ( select sum(bytes)/1024/1024/1024 redo_size from sys.v_$log ) c;