How to tell how much of the database is being used for data.

["Steve Oualline" <soualline@xxxxxxxxxxxxx>]

Title: How to tell how much of the database is being used for data.

Question: How can I tell how much free space is in the database itself?

Background: We want to keep as many records as possible in the
database as possible.  Currently we fill the database until the disk
usage reaches 80% then delete and VACUUM FULL the tables.

We'd like to just VACUUM but only VACUUM FULL is sure to
release disk space to the operating system.   However if we knew
how much space was free in the database itself, we could judge how
many new records we could dump into it.

Any help would be appreciated.