I use something like:
database sysmaster;select
name[1,8] dbspace,
sum(chksize) Pages_alloc,
sum(chksize) - sum(nfree) Pages_used,
sum(nfree) Pages_free,
round ((sum(nfree)) / (sum(chksize)) * 100, 2) percent_free
from sysdbspaces d, syschunks c
where d.dbsnum = c.dbsnum
group by 1
order by 1;