With the lack of transparency in memory foam specifications a lot of people think that memory foam as a material is all the same, slowly reacting and molding, and you pay less or more depending on brand and thickness of the mattress. Unfortunately, that couldn't be further from truth. Unlike with generic medication, where the ingredients are identical with the same effect as brand name medication, memory foam varies greatly among manufacturers in ingredients, density, manufacturing process, and thus overall quality.
Some manufacturers are more transparent than other in providing foam specifications in their brochures or online. Memory foam quality is routinely measured by its density.
Density refers to the mass of one cubic foot of foam material. Typically, this value is given in feet - 4.0 lbs or 3.5 lbs/ft³. Memory foams that contain larger amounts of polymers and less air are denser and consequently weigh more and last longer. 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