The qdb_size script will show the used space, free space and total space alloacted to the datafiles.

SQL

select sum(bytes/1024/1024/1024) “Used Space in GB” from dba_segments;
select sum(bytes/1024/1024/1024) “Free Space in GB” from dba_free_space;
select sum(bytes/1024/1024/1024) “Total in GB” from dba_data_files;

Output