Any PDB is a database and if you are considering the option of converting existing non-cdb databases into a PDB and plugging them into a 12c CDB database, the memory allocation of the CDB containing them should have that much memory allocated . For example, if I would have 2 non-cdb's each running with 700MB of RAM, I shall start the CDB instance with about 2gb of RAM (1400 MB for two instances and 600 extra for the CDB itself) and then shall monitor what is the impact over the performance. Again, this is not I shall consider as a best practice. How much each PDB wpould eventually use, I guess it would be a matter of investigation based on the workload it is incurring. I don't think that it's documented but I shall wait for Rp and others to confirm (and correct) .