select name, total_mb, free_mb, usable_file_mb,
round(((total_mb-nvl(free_mb,0))/total_mb)*100,0) percent_used
from V$ASM_DISKGROUP_STAT;
Or
SELECT G.NAME,
sum(b.total_mb) total_mb,
sum((b.total_mb - b.free_mb)) used_mb,
sum(B.FREE_MB) free_mb,
decode(sum(b.total_mb),0,0,(ROUND((1- (sum(b.free_mb) / sum(b.total_mb)))*100, 2))) pct_used,
decode(sum(b.total_mb),0,0,(ROUND(((sum(b.free_mb) / sum(b.total_mb)))*100, 2))) pct_free
FROM
v$asm_disk b,v$asm_diskgroup g
where b.group_number = g.group_number (+)
group by g.name;