Memory foam is unique because it is very high density. This makes it heavier than the average mattress, and it is also what enables it ton conform to the body's contours. Memory foam density is measured per cubic foot, so a square foot of a 5.5-pound memory foam mattress weighs 5.5 pounds. The higher the density, the better the quality, because, as stated earlier, the density is what allows it to conform to the body. A 5-pound memory foam mattress is very good. 4 is good enough, but you should never settle for less than that – otherwise the quality simply isn't there. Density is valuable when it comes to memory foam mattresses.