How to check SGA used, free, and total memory in Oracle

V$SGASTAT breaks the SGA into named pools. Summing everything except ''free memory'' gives you what is actually in use; ''free memory'' by itself is what is available for growth.

SELECT ROUND(used.bytes / 1024 / 1024, 2) AS used_mb,
       ROUND(free.bytes / 1024 / 1024, 2) AS free_mb,
       ROUND(tot.bytes  / 1024 / 1024, 2) AS 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;