Foam density measures the weight of foam per unit of volume, usually expressed in kilograms per cubic metre. It is not the same as firmness, which describes how the foam feels when pressed, since a foam can be dense yet still feel relatively soft. Density is instead one of the best indicators of how well a cushion will hold its shape over years of use.
Higher density foams, generally above about 35 kg per cubic metre, tend to resist sagging and softening longer than lower density foams, which can flatten within a couple of years under daily use. Cold foam is typically manufactured at higher densities for this reason. When comparing sofas, checking the density figure is more useful than sitting on the showroom model alone, as our buying guide explains.