Estimate the size of an RMAN database backup for new database

Database size (GB):

SQL> select sum(bytes)/1024/1024/1024 from v$datafile;

SUM(BYTES)/1024/1024/1024 

-------------------------

                887

Free space (GB):

SQL> select sum(bytes)/1024/1024/1024 from dba_free_space;

SUM(BYTES)/1024/1024/1024

-------------------------

               275

SQL>

So backup should not take more than 612 GB (887-275). But because of NULL compression the backup-piece may actually be smaller.

Reference:

Doc ID 1274720.1