Differences & Similarities Between Foam Density, Weight & Firmness

3d render of different foam mattress layers

When it comes to foam, the buying process is really simple. All you need to know is the foam type for your project, its dimensions, and its firmness. 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.

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.)LowLowLow
High Density Foam (1.9 lbs.)MediumMediumLow – Medium
Lux Foam (2.2 lbs.)GoodGoodMedium
High Resilience Foam (3.0 lbs.)Very GoodVery GoodMedium – High
Memory Foam (5.0 lbs.)ExcellentExcellentHigh
Latex Foam (5.6 lbs.)ExcellentExcellentHigh

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, Rebond, and Closed Cell foam that we sell here at Foamonline.com having an ILD 90.

Closed Cell Foam

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

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 here at Foamonline.com 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 underUsed for cribs and folding beds. Foam in this thickness is also used as mattress toppers.
Slim Mattress, Medium
6 to 8 inches
100 to 150Ideal 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 200A 10-inch mattress is 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 275A 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 want to know what foam to use for your mattress, our blog posts, “How To Choose The Right Foam Mattress” and “Which Foam Should I Use for my Mattress?” cover the subject in more depth. If you have more questions or need more information, feel free to contact us.

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