Thatâs because not all manufacturers offer exactly the same sizes in mattresses, so compare finished sizes before purchasing. A new mattress may be âdeeperâ, so look for bedding that accommodates premium quality mattresses with deeper pockets. Most standard mattresses are 9-12â deep. A premium mattress could add 4-5â to that measurement. If you add a pillowtop for more comfort, your mattress depth could easily measure 20â.