Density refers to the mass of one cubic foot of foam material. Typically, this value is given in feet - 4.0 lbs or 4.0 lbs/ft³. High density often translates into better durability and lifespan. Traditional polyurethane foams have typically lower density that memory foams because of their different molecular structure.

High-density foams contain larger amounts of polymers and less air are denser and consequently weigh more and last longer. They also react quicker – having faster response time. There are also more expensive to make since they require more raw material and less air. Low-density foams contain more air pockets per cubic foot, thus are lighter and will start collapsing much earlier, not withstanding repeated pressure over longer period of time.

Most of the memory foams currently available you can assign to one of three categories, with Low Density foam being the softest with the slowest response time:
Low Density – 2.0 lbs to 3.5 lbs

Medium Density – 3.5 to 5.0 lbs

High Density – 5.0 lbs to 8.0 lbs