We got this for the top bunk of a bunk bed we bought at Target that was a twin XL over queen.  I got the 5" so that the railing would be taller for our 7 yr old child.  We didn't want a huge mattress to reach the top of the railing and defeat the purpose of it. It works perfect for that!!!!  . She sleeps great on it.  On the ground it would even be comfortable.