How to check ASM disk group free space in Oracle

Run connected to the ASM instance, or a database instance that can see the v$asm views. Under NORMAL or HIGH redundancy, usable space is lower than free because of mirroring.

column total format 999,999 Heading "Total(G)"
column free format 999,999 Heading "Free (G)"
column Mirror_GB format 999,999 Heading "Space Used |for Mirroring(G)"
column Usable_GB format 999,999 Heading "Space Available |to Use(G)"
column pct format 999.0 Heading "% Free |in DG" 
column pct2 format 999.0 Heading "Real % Free |in DG" 
column type format a10
column name format a20
set linesize 200
set colsep '|'
prompt
Prompt "NOTE **** Incase of High or Normal Redundancy the Usable Space is lower than actual shown because of Mirroring *****"
prompt
select name,type, TOTAL_MB/1024 total, FREE_MB/1024 free, REQUIRED_MIRROR_FREE_MB/1024 Mirror_GB, USABLE_FILE_MB/1024 Usable_GB ,100-((total_MB-FREE_MB)/total_mb)*100 pct, 100-((total_MB-USABLE_FILE_MB)/total_mb)*100 pct2  from v$asm_diskgroup;