How to calculate size of tablespaces

select FILE_NAME, TABLESPACE_NAME, BYTES/1024/1024 MB, BLOCKS
from 
(
  select FILE_NAME, TABLESPACE_NAME, BYTES, BLOCKS
  from DBA_DATA_FILES
  union all
  select FILE_NAME, TABLESPACE_NAME, BYTES, BLOCKS
  from DBA_TEMP_FILES
  order by TABLESPACE_NAME, FILE_NAME
);