Differences & Similarities Between Foam Density, Weight & Firmness

3d render of different foam mattress layers

When it comes to foam, the buying process is simple. However, there can be potential confusion when it comes to a foam’s density, weight, and firmness. From a basic perspective, these three terms are understood. However, when applied to the foam industry, the meanings of these terms can be altered to better fit the needs of the industry. As such, there can be some questions and misunderstandings as to what these terms mean along with their relationship and differences. This article is intended to alleviate any questions and misunderstandings that you may have.

Foam Density

Foam density is the mass of a foam’s material within a cubic foot. The denser the foam, the more compact the foam material is. This, in turn, makes the foam heavier. A foam’s density is characterized by pounds and is determined by taking its mass and dividing it by its unit volume.

Polyurethane foam stacked on top of each other

Polyurethane Foam – the lowest density foam we offer here at Foamonline.com at 1.2 lbs.

Let’s use an example. In the foam industry, foam is measured in pounds per cubic foot. As such, we take a cubic foot of Memory foam and measure it out. It weighs 3 lbs. This 3 lbs. foam weight is then divided by its unit volume – in this case, 1 cubic foot. 3 divided by 1 equals 3. As such, the memory foams density is 3 lbs.

Another example. If the memory foam was 10 cubic feet and weighed 30 lbs., it would be divided by the unit volume, which is now 10 cubic feet, and the foam density would still be 3 lbs. This example also aids with confusion over foam density and foam weight. There is often interchangeability between foam density and foam weight that can lead to confusion. Per this example, the memory foams density or material weight is 3 lbs. and its overall weight is 30 lbs. That is why it can be important to distinguish between the density or material weight of foam and its overall weight.

Importance of Foam Density

Foam density is important because it is connected to the quality and durability of foam. For example, a Lux foam mattress with a foam density of 2.2 lbs. would not be as comfortable or long-lasting as a Latex foam mattress with a density of 5.6 lbs. This is because there is more compressed foam material in Latex foam than there is in Lux foam. As such, a Lux foam mattress will deteriorate more quickly, leading to sagging, indentations, and a general lessened state of comfort over time. That is not to say that Latex foam would be better used for all applications. While Latex foam would be better used as a foam mattress, Lux foam would find better usage in couch cushions, automobile seating, or in a multi-layered mattress.

Perhaps this comparison chart can give a better visual understanding of different foam types and their relationship to density:

Foam Type (Density)
Polyurethane Foam (1.2 lbs.) Low Low Very Low
High Density Foam (1.9 lbs.) Medium Medium Low
Lux Foam (2.2 lbs.) Good Good Medium
High Resilience Foam (3.0 lbs.) Very Good Very Good High
Memory Foam (5.0 lbs.) Excellent Excellent Very High
Latex Foam (5.6 lbs.) Excellent Excellent Very High

Foam Weight

Foam weight is the physical weight of the foam. So, if you purchase a 10 cubic foot piece of foam at a density of 5 lbs., the weight of that foam would be 50 lbs. (50 = 10 cubic feet X 5 lbs. density). The foam density remains 5 lbs. Keep in mind that foam mattresses can become quite heavy as you increase the thickness and size, especially when the foam is dense. Plan accordingly if it is.

Foam Firmness

Foam firmness is how much a foam compresses when pressure is applied to it. Firmness is a simplified way of saying Indent Load Deflection (ILD). All foam types have an ILD rating. To get an ILD rating, a 15” x 15” x 4” foam block has pressure applied to it by a circular disk measuring 1’ in diameter and 4” thick. The ILD rating is achieved when the circular disk compresses the foam by 25% of its thickness, in this case 1”. The amount of weight applied to reach this target percentage is the ILD number that the foam receives as its firmness.

For example, if 50 lbs. of pressure is required to compress a block of foam 25%, the foam is given an ILD 50, which is firm. More force is applied to firmer foam types and less force is applied to softer foam types. The higher the ILD number, the more force is applied, and thus the firmer the foam. Most foams have an ILD between 15 and 70. However, foam can go even higher, with Charcoal and Closed Cell foam that we sell here at Foamonline.com having an ILD 90 and Rebond foam having an ILD 150.

Closed Cell Foam

Closed Cell foam – it might look dense but is a 2 lbs. density.

Foam Density vs. Firmness

There is also confusion surrounding foam density and foam firmness. Many people think that a higher foam density means a firmer foam. This is not the case. A low-density foam can be firm and a foam that is high in density foam can be soft.

If you view our ILD Chart, you can see that the foam we offer come in many different foam firmness’s. Foam firmness’s range from very soft to extra firm. For example, we offer High Resilience foam in both a 2.8 and 3.0 lbs. density that is also available in most all firmness’s. As such, foam density has no correlation with foam firmness. Foam firmness has nothing to do with the quality or longevity of foam. It is simply used to determine how much a foam compresses when pressure is applied.

When purchasing a foam mattress, some of the characteristics chosen such as density and thickness is budget-oriented while firmness is mostly a personal preference. However, an important aspect that should influence one’s decision is the weight of the user/s. For mattress density and thickness, the heavier the person, the denser and thicker the mattress should be to support their weight. If the mattress is low density, it will lack support and lead to excessive sagging. If the mattress is too thin, it can bottom out and cause excessive pressure points. This is most notable when sleeping on your side.

The lighter the person, the less dense and thinner the mattress can be to support their weight. Lighter people do not need the same conformity from higher density foam to provide pressure relief that heavier people need. A thicker mattress could be purchased but is not a cost-effective purchase.

The chart below can give you a better understanding of what mattress thickness is appropriate for you:

Mattress Thickness, Firmness
Physical Weight (pounds)
Low Profile Mattress, Medium Soft
2 to 5 inches
100 and under Used for cribs, folding beds, and mattress toppers.
Slim Mattress, Medium
6 to 8 inches
100 to 150 Ideal for people that have difficulty getting in to and out of bed, as well as children.
Standard Mattress, Medium Firm
8 to 10 inches
150 to 200 The typical mattress size found on the market. It usually consists of a couple of different foam layers for comfort.
Standard Mattress, Firm
10 to 12 inches
200 to 275 A 12-inch mattress will typically have shock-absorbing foam as the bottom layer and transitional layers of various other foam types.
Thick Mattress, Etra Firm
12 + inches
275 + Mattresses in this thickness have various layered foams and other materials. However, as height increases, convenience decreases.

With mattress firmness, the heavier the person is, the firmer their mattress should be. A firmer firmness will provide ample support to pressure points for side sleepers and general comfort. Conversely, if a mattress is double digits in thickness and is a soft firmness, a heavy user will sink into their mattress and will have difficulty getting out. A lighter person will want a softer firmness. We recommend medium to medium-soft. Lighter people exert less pressure, so a softer mattress will conform to their body shape better and provide more support to their pressure points.


If you need more information about specific types of foam, visit our Foam Types page. If you have more questions or need more information, feel free to contact us.

11 replies
  1. Thomas
    Thomas says:

    We are looking for foam for an application we have in the neonatal intensive care units and delivery rooms of hospitals.  We are looking for a foam piece with the dimensions of 623mm x 467mm x 40.5mm.  
    ILD of 9lbs at 25% and density of ~3-5 lbs. 
    We are prototyping at low quantities to begin with to make sure the foam works then we will scale up making 22k of these beds. Please let me know if you have anything in this range please.  Thanks! 
    Thank you! 

    • FoamOnline
      FoamOnline says:

      Hi Thomas,

      The most commonly used foam for infants is a medium quality 1.9 lb High Density in medium-firm ILD 36. A lower quality 1.2 lb Polyurethane foam will not be sufficient because the foam will take to the shape of the infant’s body and will not provide the proper support. It will soften and indent in the center area while remaining firmer around the edges. You have referenced an excellent quality 3 lb High Resilience type foam in an ILD 40 that can also be used in this type of application. There is a third good foam type called Lux at 2.2 lbs in an ILD 36 that is also within the range of your requirements.

  2. Stephen
    Stephen says:

    Hi. I’m looking to make seat cushions for the bucket seats in my car. They would need to be no more than a half inch thick so head room is not compromised. I would need a very high firmness foam so that it wouldn’t bottom out. What would be the firmness I should aim for to make the seats just a bit softer? Currently I make due with just sitting in the bare shell, so any cushioning is going to be an improvement. But I understand there are some foams that are extremely dense and wouldn’t make much of an improvement. I am about 240 pounds. Thank you.

    • FoamOnline
      FoamOnline says:

      The issue is that thinner foam is less firm than thicker foam. Half-inch thick foam is very thin and will bottom out easily at 240 lbs. If you are going this route, then our recommendation would be to use half-inch thick Rebond foam at an ILD 150 (Extra Firm). That is the firmest foam we offer.

  3. John Platos
    John Platos says:

    Hi I’m looking to make a reflex boxing bag that will need to withstand constant hard punches without deforming. I’d like to use a very firm foam core. Do you have any recommendations? Thank you.

    • FoamOnline
      FoamOnline says:

      Hi John. There are two foam types that will work. The first foam type is known as Rebond, and is very heavy at 5 lbs per cubic foot and very hard in an ILD 150. The second foam type is known as High Resilience, and is heavy at 3 lbs per cubic foot and hard in an ILD 70. The Rebond type foam comes in the firmness extra firm only and the High Resilience type foam comes in a variety of firmnesses, ranging from very soft to extra firm. Both of these foam types are of excellent quality.

      You can price quote or order either of these foam types by following this link: https://foamonline.com/cube/

  4. Andrew N Dart
    Andrew N Dart says:

    I need to repad 12 wooden prayer kneelers (prie Dieus) in a Eucharistic Adoration Chapel. They get a lot of use. Which foam do you recommend I buy. I have no experience.
    Peace, Andy

    • FoamOnline
      FoamOnline says:

      Hi Andrew. The foam type used in wooden prayer kneelers for longevity and durability is High Resilience. Being used as a kneeler with a large amount of weight placed on the foam per square inch, we would recommend that the firmness be extra firm. We would recommend that the foam be wrapped in 3/4 inch thick Dacron to round the edges of the foam as the cover is pulled down tightly.

  5. Cynthia Larocque
    Cynthia Larocque says:

    I am completely re-doing a sectional, and I selected thick High Resilience Foam in a Medium Firm as a base since I have a much heavier partner who needs the extra support in the cushion. Unfortunately, my butt goes numb sitting on the Medium Firm since I am small. Since I haven’t made the cushion covers yet and the cushion height can be anything, I’d like to layer the cushions with a top layer of foam (also High Resilience) in either Very Soft or Medium Soft for a soft sit for me, and I was wondering if you could help me with a few questions: Which would you recommend, and in what thickness to go over top the Medium Firm foam for a much softer sit? Also, do I need to layer on both sides of the foam? Thank you in advance!

    • FoamOnline
      FoamOnline says:

      Hi Cynthia. First, we would recommend Medium Soft for your cushion. Very Soft will likely sink down too quickly. Second, the foam should ideally be a 2″ Medium Soft top layer. If the cushion is reversible, you could have 1″ Medium Soft on both the top and the bottom of the cushion. Keep in mind that the average height of a sectional from the floor to the top of the cushion after it is reupholstered should be 18″.


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