dba_free_space

Static view dba_free_space, mainly used for calculating the free space in any tablespace  (excluding temporary tablespace) will not display any row if tablespace is 100% used. 

Below is the test for undo tablespace, that is 100% utilized.

SQL> select file_id,bytes/1024 from dba_data_files where tablespace_name='UNDOTBS';

   FILE_ID BYTES/1024

---------- ----------

         3    8192000

        61    8192000

        62    8192000

        

SQL> select file_id,bytes/1024 from dba_free_space where tablespace_name='UNDOTBS';

no rows selected

SQL>