The following query if run will fetch Free space, Used space and Total space of each tablespace available.
SELECT Total.name "Tablespace Name",
Free_space,
(total_space-Free_space) Used_space,
total_space
FROM
(select tablespace_name, sum(bytes/1024/1024) Free_Space
from sys.dba_free_space
group by tablespace_name
) Free,
(select b.name, sum(bytes/1024/1024) TOTAL_SPACE
from sys.v_$datafile a, sys.v_$tablespace B
where a.ts# = b.ts#
group by b.name
) Total
WHERE Free.Tablespace_name = Total.name;
This tip comes from Lazydba
