Query To Check Tablespace Allocated Size and Free Size in GB

SQL> select b.tablespace_name, tbs_size SizeGB, a.free_space FreeGB from
    (select tablespace_name, round(sum(bytes)/1024/1024/1024,1) as free_space
    from dba_free_space group by tablespace_name UNION
    select tablespace_name, round((free_space)/1024/1024/1024,1) as free_space from                                         dba_temp_free_space) a,
    (select tablespace_name, sum(bytes)/1024/1024/1024 as tbs_size
    from dba_data_files group by tablespace_name UNION
    select tablespace_name, sum(bytes)/1024/1024/1024 tbs_size
    from dba_temp_files group by tablespace_name ) b where a.tablespace_name(+)=b.tablespace_name;

TABLESPACE_NAME                    SIZEGB               FREEGB
------------------------------ ---------- ---------------------------------------
SYSAUX                                            2                             1.4
SYSTEM                                            2                             1.3
TEMP                                                 2.34472656            2.3
UNDOTBS1                                       2                             2
USERS                                              36.5344238            14.7


Happy Learning ...