Hi brass,
That depends on how you define “feel” which is very subjective and can vary with all the properties and specs of the foam (such as resilience, point elasticity, cell structure, and others) which are “built in” to the chemical formulation of the polyfoam (regardless of density).
Assuming though that you are only talking about the firmness/softness of a foam … then the IFD rating for polyfoam is measured by using a 50 sq inch round compressor foot and compressing a 4" piece of foam of at least 15" x 15" (usually 20" x 20") by 25% of its thickness (so a 4" piece is compressed to 3") and after a short waiting period measuring the force needed to keep it compressed. There is 1 lb of force applied first to even out any irregularities in the foam surface.
This means that IFD is an accurate representation of the relative firmness of 4" of a foam material compared to others that are measured using the same testing protocols and that are compressed by one inch. IFD testing will usually also be a reasonable prediction of the relative firmness of a foam in other circumstances or applications as well.
There are also other factors though that can affect how soft or firm a foam feels other than IFD. The most important of these is compression modulus which is the rate that a foam becomes firmer as you compress it more deeply. Foams with a higher compression modulus (also called support factor, sag factor, or comfort factor) will feel firmer when they are compressed more than 25% than a foam with a lower compression modulus. The compression modulus of a foam will also depend on the chemical foam formulation although with the same chemical foam formulation higher density will usually have a higher compression modulus. HR foams or “high comfort” or “high performance” foams will generally use a chemical formulation that has a higher compression modulus than conventional foam formulations (regardless of density). There is more in post #4 here about the various factors that can affect how firm or soft a foam feels and performs (other than its position in a mattress and the layers above or below it which are not part of the properties of the foam itself).
Any density of foam can be made in a very wide range of IFD’s and density is more connected to durability not firmness but IFD itself isn’t the only factor that is involved in how firm a layer of a foam “feels”.
Phoenix