tablespace usage

Hi All

We are having issue while finding exact space used in our database.  Could please help us in providing  query which gives the correct output ?

For the below query to check tablespace,

select b.tablespace_name, tbs_size SizeMb, a.free_space FreeMb

from  (select tablespace_name, round(sum(bytes)/1024/1024 ,2) as free_space

       from dba_free_space

       group by tablespace_name) a,

      (select tablespace_name, sum(bytes)/1024/1024 as tbs_size

       from dba_data_files

       group by tablespace_name) b

where a.tablespace_name(+)=b.tablespace_name;

We are getting for tablespace XXSOA_TS_DATA only 2.5 GB free. usage as 175GB, Total allocated is 180GB.