How Does Mattress Firmness Work?
A prime myth of foam mattress firmness is that density is what makes a visco mattress soft or firm. This is not true. You can have very soft or very firm high density viscoelastic foam. This is emphasized by the Polyurethane Foam Association (PFA) themselves on their website in the Density Standards and Guidelines section.
Firmness is, in fact, determined by the ILD rating (Indentation Load Deflection), a property all foams have. Industry experts actually use IFD (Indentation Force Deflection) to measure firmness, which is about the same thing. Here's how it works.
IFD/ ILD measures the amount of force, in pounds, required to indent a 50" inch disc into a 15"x15"x4" viscoelastic foam sample and make a 1" indentation. For example, 14 ILD means it would take 14 pounds to make a 1" indentation in the foam.