Density is a factor that effectively determines how a mattress will feel, perform and last. Density rating is a measurement of how much visco-elastic memory foam is in the top layer, or comfort layer, of a mattress. A higher density memory foam, at an optimum depth and working with other premium material layers, delivers outstanding support and pressure relief. It also lasts longer. Lower density memory foam cannot provide the same performance or comfort, and also wears out faster. Ergoflex mattresses have the highest rating of true high-density memory foam in this price range, by a considerable margin. Read more on our Buyers Guide.