Calculate free space of a tablespace

The following script calculates the free space of a tablespace in an Oracle database:

SELECT TABLESPACE_NAME,
(SUM(BYTES)/1024) FREE_KB,
(SUM(BYTES)/(1024*1024)) FREE_MB
FROM DBA_FREE_SPACE
GROUP BY TABLESPACE_NAME;

DBA_FREE_SPACE describes the free extents in all tablespaces in the database.