Oracle: Calculate free space in tablespaces

select
   tablespace_name,
   round(total_space_gb, 1)  total_gb,
   round(free_space_gb , 1)  free_gb,
   to_char(100 * free_space_gb / total_space_gb, '990.0' ) || ' %' free_pct
from
(
    select
      ts.tablespace_name,
      ts.total_space_gb,
      ts.total_space_gb - df.used_space_gb  free_space_gb
   from
      (
        select
           tablespace_name,
           sum(bytes) / 1024/1024/1024 as total_space_gb
        from
           dba_data_files
        group by
           tablespace_name
      )                                                     ts left join
      (
        select
           tablespace_name,
           sum(bytes) / 1024/1024/1024 as used_space_gb
        from
           dba_segments
        group by
           tablespace_name
      )                                                      df on df.tablespace_name = ts.tablespace_name
)
-- where
--     tablespace_name = 'TS_XYZ'
;