Density matters perhaps more than any of other single factor in finding the right memory foam mattress.
Each memory foam mattress is measured in pounds of density – three, four, five pounds or more for example. Density is merely a number that corresponds to the weight of the foam; however, this figure is not representative of the total weight of the mattress. When a mattress is characterized as having four pounds of density that means every cubic foot of foam weighs four pounds.
This figure provides the consumer a quick indicator of how firm or soft a mattress is going to be.
A memory foam mattress of three-pounds or less is not generally recommended because the material will likely be too soft to properly support the user’s body. A red flag is also raised regarding quality when a memory foam mattress has less than three pounds of density. It could be a sign that the foam is a cheap variety that isn’t likely to last – this is especially true of memory foam mattresses that are not manufactured in the United States. 
While memory foam is also available in density increments greater than five pounds, these too are generally less desirable because they tend to be too firm for most tastes.
For many users, the memory foam sweet spot falls in the middle where density ranges from three to four pounds. At this density, the mattress is firm enough to provide the maximum amount of support but isn’t so soft that the user feels as if they are melting through to the foundation.
Ultimately, the proper density of a memory foam mattress will depend on personal preference. Side sleepers are more likely to prefer a medium-density memory foam mattress that will allow their shoulders – that are bearing more weight than the rest of the body – to be absorbed for maximum comfort. Back and stomach sleepers may find that firmer mattresses are more desirable for the greater support provided.