How much space each user is consuming

SELECT u.NAME Owner,
t.NAME Tablespace,
SUM((seg.blocks * t.BLOCKSIZE)/1024)/1024 MB
FROM SYS.ts$ t,
SYS.user$ u,
SYS.seg$ seg
WHERE seg.user# = u.user#
AND t.ts# = seg.ts#
GROUP BY u.NAME, t.NAME