The actual thickness is one of the first things that make the difference between one memory foam mattress and another. You might run into seven inch thick mattresses, as well as 15 inch thick models. Keep in mind that most models are not entirely made of memory foam. In fact, they only come up with a memory foam top, while the bottom part is made of something else. Consider your sleeping needs and preferences then. Remember that the thicker a mattress is, the firmer it becomes. Check www.wakeupinabetterway.org for some detailed reviews if you are not sure about one mattress in particular.