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;