When you start looking for a new mattress, you quickly realize that there are dozens of mattress options just one click away. Many people become overwhelmed and give up on comparison shopping. But with a little bit of background, you can help understand two of the most commonly discussed characteristics for foam mattresses — density and firmness.

What is foam density?

At its most basic level, density tells you about the physical properties of the mattress. But as a consumer, the measurements you’ll see listed for mattress density refer to its weight and to its compressibility.

How is density measured?

It’s pretty simple. Density is a measurement of weight by volume. This is recorded as pounds per cubic foot (PCF).

Pounds Per Cubic Foot (PCF)

PCF is a straightforward measurement: take a cube of foam with one foot sides, weigh it, and you have the PCF for that foam.

Why does PCF matter?

In general, foams with higher PCF are more likely to be durable with regular use. They tend to resist sagging or indenting more than foams with low density.

In addition, higher PCF foams will often have a firmer feel. Of course, this isn’t universally true, but as a general starting point in evaluating mattresses, you can expect foams with higher densities — measured in PCF — to have more firmness.

What are the limits of evaluating PCF?

PCF tells you how much the foam weighs, and weighing more isn’t the be-all, end-all. Anyone could manufacture a foam that was just simply heavy. We value foams for other characteristics such as the support they provide, how they respond when pressure is applied and removed, and how well they breathe just to name a few.

Also, it is important to remember that most mattresses are constructed using multiple layers of foams. A high-density foam placed on top of another layer of high-density foam may have a very different feel than a high-density foam layer on top of a low-density foam layer. This gets even trickier when other layers — such as innerspring coils — are involved.

What a common range?

For memory foam, which is usually used in the top of mattresses as comfort layers, you can keep these general principles in mind:

  • Less than 3 PCF: low-density, greater likelihood of wear, often softer with more sink.
  • 3-5 PCF: medium-density, generally considered to be durable, some sink and contouring but balanced with firmness and support.
  • 5+ PCF: likely very durable but also commonly very firm with only minor sink.

For support layers, which are typically made with polyurethane foams, the focus is less on firmness and more on durability. For these foams, keep these general principles in mind:

  • 1 PCF or less: very light foam, may be prone to losing shape or strength
  • 1-1.8 PCF: low density, can be a sign of weaker or low-quality foam
  • 1.8 PCF or higher: average or higher density, more likely to offer durable support.

For latex foams, which naturally have more bounce and resilience than memory foam, the range is usually between 60 kilograms per cubic meter (kg/m3) to 90 kg/m3. With latex, this correlates more directly to firmness than with memory foam. Usually, you are more likely to find the ILD reported for latex. Read on for more about ILD.

Does density mean the same thing as firmness?

No. As we explained above, in support foams and memory foams, density is only a measurement of weight. While that may often correlate with firmness, it is not the same thing. If we really want to know about firmness, we need a different measurement — indentation load deflection (ILD).

Indentation Load Deflection (ILD)

First things first, ILD or IFD?

For all practical purposes for the everyday consumer, indentation load deflection (ILD) is the same thing as indentation force deflection (IFD).

What is ILD?

ILD is a way of measuring the compressability of a foam. This is done with a standardized process in a lab: a machine puts a certain amount of pressure on a round disc, and they measure how many pounds of force it takes to indent a rectangular piece of foam 25% of its original height.

In very rare cases, an ILD may also be reported for the pounds of force needed to indent the foam to 65%. It may also be called the support factor. While this is usually measured by mattress companies in internal testing, it is rarely shared with potential customers.

Why does ILD matter?

ILD gives you insight into how the foam responds to pressure and by extension, a sense of the foam’s firmness. If a foam has an extremely low ILD, it will be very responsive to your weight on the mattress. You are much more likely to sink into a mattress with a low ILD.

What are the limits of evaluating ILD?

There are a few challenges in examining ILD when mattress shopping.

First, in real world applications, the ILD is affected by the thickness of the foam. It takes more pressure to indent 4 inches of foam than 3 inches, so a 4 inch piece of foam and 3 inch piece of foam with the same ILD won’t have the same actual firmness. The 4 inch piece of foam will actually feel firmer.

Second, ILD as commonly reported in the mattress industry only speaks to the top 25% of the foam. It doesn’t provide an indication of the total level of compressability for the piece of foam as a whole. And it also doesn’t account for how different foams or other materials may be layered within a mattress.

Third, ILD doesn’t address resilience of the foam and how quickly it bounces back to its original shape, which can dramatically affect the feel of the mattress. Some foams, such as latex, have a reputation for springing back more quickly than memory foam.

What’s a common range?

For memory foams, a common range for ILD that you can look for is outlined below:

  • 10 and under: very soft, lots of sink
  • 10-12: soft, quite a bit of sink and contouring
  • 12-14: medium, some sink and contouring
  • 14-16: medium firm, minor sink
  • 16+: very firm, limited sink

For support foams, it is harder to state the expected ILD, but because these foams are not designed to provide direct comfort, you may see much higher ILDs reported.

What about ILD ranges for latex?

In general, latex foams have more firmness and springiness than memory foams. When looking at ILD for latex, it’s not a direct comparison with other foams. For latex, a general reference range is

  • 15-25: very soft to soft
  • 25-35: medium to medium-firm
  • 35+: firm to very firm

How else is firmness described?

Another, simpler way of evaluating firmness is on a scale of 1-10. This scale is commonly used by mattress companies and by reviewers. In this scale, a 1 is almost as soft as a pillow while a 10 is nearly as hard as a rock. You can find this scale described in detail in our Mattress Firmness Guide.

How can knowing the density help me choose a mattress?

We suggest that for any mattress that you’re seriously considering, you look for a listing of the foam density for each layer and go through this process:

Look at the PCF and/or ILD of the comfort layer. If you notice a density or an ILD that seems very high or very low, make a note of it and ask:

  • Does this fit with my comfort preferences?
  • If not, does some other feature or aspect of the mattress design account for this outlier?
  • What type of material is it, and what are the typical performance qualities of that material?

Look at the PCF of the largest support layer. If this number seems low, proceed with caution. Ask yourself:

  • How long am I hoping this mattress will last?
  • How important is durability for me relative to other factors (such as cost, materials, etc.)?
  • Will this mattress get heavy use (in terms of weight of sleeper(s), sexual activity, etc.)?
  • What are the details of the mattress warranty and the company’s reputation for customer service?

Evaluate the PCF and/or ILD of any other foams used. If any seem to be abnormally high or low, ask yourself:

  • Does this make sense within the overall mattress design?

In other words, as you go through this process, be on the lookout for red flags. For example, maybe you notice a really low PCF support foam or a comfort layer with extremely high ILD. If so, look for other meaningful data points. Ask if that makes sense given what you’re looking for. Check out our reviews or other trusted review sites to see if people have reported problems that might be related to this. Ask yourself if there’s a design element intended to give the mattress a certain feel that might be involved. If you’re worried about durability, look into the company’s track record, reputation for customer service, and mattress warranty.

What are the limits of using density in comparing mattresses?

Remember that density doesn’t tell us everything we want to know. While it can help determine the likely characteristics of a mattress, it probably won’t be able to tell you for sure which mattress is the best for you. Thankfully, almost all of the direct-to-consumer mattress companies offer sleep trials, so you can use density and firmness information to narrow your choices down and then try out a mattress in your own home.

How can I find out the density and firmness of mattress foams?

There are three ways to find the density of the foams used in a mattress:

  • Check out our guides and reviews. We always post this information when it is publicly available.
  • Look at the manufacturer website. It varies, but many mattress sellers include this information in their mattress description or FAQ.
  • Contact the manufacturer directly. Even if it’s not on their website, you may be able to call or e-mail the company to find out.

What does it mean if a company doesn’t state the density and/or ILD?

It’s hard to say exactly what to make of it when the density and firmness isn’t published. This is a competitive industry, and some companies may treat this as proprietary business information.

That said, it is OK to be skeptical of a mattress whose specifications you don’t know. Not having this information hurts your ability to comparison shop, and fairly or not, makes it seem like perhaps the company is trying to hide something. When companies clearly share their specifications, it helps to know that they aren’t cutting corners by using low-quality components.

Where can I read more?

by: Eric Seger