Current sga usage - oracle database script

SELECT round(used.bytes /1024/1024, 2) used_mb,
       round(free.bytes /1024/1024, 2) free_mb,
       round(tot.bytes /1024/1024, 2) total_mb
FROM
  (SELECT sum(bytes) bytes
   FROM v$sgastat
   WHERE name != 'free memory') used ,
  (SELECT sum(bytes) bytes
   FROM v$sgastat
   WHERE name = 'free memory') FREE,

  (SELECT sum(bytes) bytes
   FROM v$sgastat) tot /