As others have said, it depends on the 'drop' you want on the sides/ends. Mattresses today are so thick it's just silly. Mine is 20+ inches thick - read really, really, expensive sheets! If it's for someone else, you'll need to know that info if it's important to you to cover the mattress and/or box spring. Then there's the half-height box springs for the giant mattresses...wish we had gotten one of those so I didn't need a step ladder to get into bed!