Foam Density vs Hardness vs Resilience: What Each Number Tells You

A foam spec sheet usually leads with three numbers: density, hardness and resilience. Buyers often read them as three ways of saying the same thing, that a “good” foam is heavy, firm and bouncy. It isn’t that simple. Each number measures a different property, each comes from a different test, and they can move in opposite directions. This article explains what foam density vs hardness vs resilience actually tells you, and how to read the three together. For the full list of foam tests and standards, see our foam testing guide.

Three numbers, three different questions

The quickest way to keep them apart is to ask what question each one answers:

• Density: how much material is in the foam?
• Hardness: how much force does it take to compress it?
• Resilience: how much energy does it give back when you release it?

Foam density: the material behind every other number

Density is mass divided by volume. Most markets quote it in kg/m³, while North American buyers often use lb/ft³; one lb/ft³ is about 16 kg/m³. A 1.8 lb/ft³ foam therefore sits around 29 kg/m³.

Density matters because it sets the ceiling for durability. More polymer per volume usually means thicker cell walls, and thicker cell walls hold their shape longer under repeated load. However, density says nothing directly about how the foam feels. Two foams at 30 kg/m³ can be soft and firm respectively, depending on the chemistry. Measurement details, sample conditioning and common errors are covered in foam density test standards.

Foam hardness: what IFD and ILD tell you

Hardness is measured by pressing a flat circular indentor into a sample and recording the force needed to reach a set percentage of its thickness. In the US bedding trade this is called IFD (indentation force deflection), and older documents call it ILD. ASTM D3574 Test B1 usually reports IFD at 25% and 65% indentation. The ratio between the two, 65% divided by 25%, is the support factor: a higher ratio means the foam pushes back harder as you sink deeper. The full ASTM procedure and how to compare IFD values are covered in IFD vs ILD.

ISO 2439 approaches the same property with several methods. Its method A reports the force at 40% indentation after a 30 second hold, while method B records values at 25%, 40% and 65%. Because indentation level, hold time and sample thickness differ between the methods, an ISO number and an ASTM number for the same foam won’t match. Always check which method a value came from before comparing two suppliers.

Foam resilience: how much energy comes back

Resilience is measured with the ball rebound test. A steel ball drops from a fixed height onto the sample, and the rebound height is reported as a percentage of the drop height. High resilience (HR) foam returns a large share of the energy and feels lively. Viscoelastic memory foam returns very little on purpose, which gives it its slow, sinking feel.

A high number isn’t automatically better. Premium mattress and seating foams target high rebound, but packaging and memory foams deliberately aim low. What matters is that the value stays within the agreed range from batch to batch. The method, sample preparation and typical errors are explained in what is foam resilience testing, and the measurement itself is made with the MK-1040 Foam Resilience Tester.

Why density, hardness and resilience don’t move together

Foam chemists can adjust hardness and resilience largely independently of density. Changing the polyol type, the isocyanate index, filler content or the cell structure shifts firmness and rebound while the mass per volume stays the same. The production side of this is covered in how polyurethane foam is made. As a result, you’ll find:

• low-density foams made firm with additives, which feel supportive at first but lose thickness quickly in use;
• high-density memory foams that feel soft and show very low rebound;
• HR foams with moderate density but high resilience and a high support factor.

That’s why a specification that lists only density leaves most of the important questions open. A complete spec names all three values and the standard each was measured to.

Reading a foam spec sheet

When you compare offers, read the three numbers in order. Start with density to judge material content and likely service life. Then check hardness, and confirm whether it’s ISO 2439 at 40% or ASTM IFD at 25%, since mixing them up is the most common comparison error. Finally, look at resilience to see whether the foam’s character suits the product. If durability is critical, ask for dynamic fatigue data as well, because none of these three numbers shows how the foam ages.

Frequently asked questions

Does higher density mean firmer foam?

No. Density measures material content, while firmness comes from the formulation. A high-density foam can be soft and a low-density foam can be firm.

What is a good IFD for a mattress?

There’s no single right value; it depends on the layer and the intended feel. Comfort layers use lower IFD values and support cores use higher ones. Compare values only when they were measured to the same method.

Is high resilience foam better than memory foam?

Neither is better in general. HR foam suits applications that need quick recovery and support, while memory foam is chosen for pressure relief and a slow response.

Which test shows how long foam will last?

Density gives a first indication, but the direct answer comes from a foam fatigue test, which measures thickness and hardness loss after repeated loading.