Density & Quality - Direct Correlation?

First, let me start by saying that I’ve learned a lot from the site. Thank you for all the great info!

After reading through much of the materials provided here, I see that the density of the foam as well as the support/base layers seems to be the most important factor in choosing a mattress, and the ILD is usually not too relevant. I’m still confused by a few things:

  1. When assessing the “quality” of a foam mattress, is there such a thing as “premium” medium density foam and “low-grade” high density foam, i.e. would it be possible for a 3 lb foam be “better” than a 5 lb foam because the “quality” of the former is better? I see some high-priced mattresses with only 3 lb foams, and I can’t help but wonder if their foam is somehow “different” than the cheaper alternatives.

  2. Is density always directly correlated to firmness, i.e. should a 5 lb mattress always feel firmer than a 3 lb mattress? I’ve seen many descriptions of mattresses that say they’re “plush” but the stated density of the foam is 4 lbs. Yet, on other mattresses with lower density, I see people complaining that it’s “hard as a rock”. How do you make a high density mattress feel “plush” and a low density one feel “firm”? I see some popular online brands have “refreshed” their mattresses based on customer feedback so that they’re now “softer” beds. Does this mean that they have reduced the density of the foam to achieve this? This is important to me because if I decide to purchase a mattress unseen, I’d rather go with the specs than the subjective feel of reviewers.

I recently purchased a cheap memory foam mattress for a guest room, and I weighed the darn thing before opening it up. Based on my calculations, the claim that it has 4 lb memory foam and 1.8 lb base foam seems valid, but it felt way too soft for me. If the firmness of memory foam is fairly consistent with its density, then I know that I will have to definitely purchase higher grade of foam. BUT, again, I see a lot of people complaining about some mattresses being too firm when they’re only 3 lbs…@@

Hi sliceoflife,

When you sleep on a mattress the upper layers and components in the mattress will compress and deflect more than the deeper layers or components partly because the comfort layers are usually made to be softer than the deeper transition and support layers or components of a mattress (and firmness/softness is also a factor in the durability of a material) and partly because they are closer to the sleeping surface and subject to direct compression without any layers above them absorbing some of the compression forces first. It’s this constant deflection of the materials in the upper layers of the mattress that softens and breaks down the foam materials and leads to the loss of comfort and support of the mattress over time.

What this means is that the upper layers of a mattress are generally the weakest link of a mattress and will usually have a bigger effect on the durability and useful life of a mattress than the deeper layers in a mattress.

The density of polyfoam and memory foam is the single biggest variable that affects the durability and useful life of the material and density isn’t directly related to firmness. Any specific density of memory foam or polyfoam can be made in a range of firmness levels although firmness/softness is a secondary durability factor as well because softer layers will compress and deflect more and be less durable than firmer layers of the same material.

With latex density is directly related to firmness/softness so a latex layer that is the same type and blend of latex as another layer and is the same density will generally have a similar firmness as well but different types and blends of latex can have different firmness levels for the same density so this only applies to comparing density and firmness with the same type and blend of latex. Durability isn’t an issue with latex because any type or blend of latex would be a durable material relative to other types of foam although they can have different properties and a different “feel”. There is more information about the many variables that can affect durability in the durability guidelines here.

Strictly in terms of durability … the chemical formulation of memory foam and polyfoam can have “some” effect on durability but this isn’t something that I would take into account because density is the single biggest factor and is what I would use when comparing the materials in a mattress to the durability guidelines to identify any lower quality materials or weak links in a mattress. Once again though … the density has little to do with the firmness or “feel” of memory foam or polyfoam because outside of durability there are a wide range of different properties that can be formulated into different foam materials such as firmness, resilience, temperature sensitivity, point elasticity, and compression modulus (the rate that a foam becomes firmer as you sink into it more deeply) with different versions of the same type and density of foam.

In other words … with memory foam and polyfoam density is a “quality/durability spec” and firmness or ILD/IFD is a “comfort spec”. Comfort specs will have more to do with how well you sleep on a mattress and durability specs will have more to do with how long you sleep well on the same mattress.

I would always keep in mind that there are no “standard” definitions or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others so a mattress that one manufacturer rates as being a specific firmness could be rated very differently by another manufacturer. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person can feel like “medium” for someone else or even “soft” for someone else (or vice versa) depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress as well (see post #15 here) so different people can also have very different opinions on how two mattresses compare in terms of firmness and some people may rate one mattress as being firmer than another and someone else may rate them the other way around. This is all relative and very subjective and is as much an art as a science.

You can assess a mattress for durability based on the type and quality of the materials and components inside it but the only way to know for certain how a mattress will “feel” to you in terms of comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) and how well you will sleep on it will be based on your own careful testing (hopefully using the testing guidelines in the tutorial) or your own personal experience when you sleep on it regardless of anyone else’s experience on the same mattress.


Thank you very much, Phoenix.
I feel much better now about where I should focus on my own mattress shopping in terms of foam. I am also exploring latex - perhaps that could be the answer for me as my preference for firmness and less of a “contouring” feeling. I’ve learned that even a highly dense foam can have an “enveloping” feel which I don’t particularly like. Thanks again!

Hi sliceoflife,

Assuming that the materials in a mattress you are considering are durable enough for your body type and meet the durability guidelines here relative to your weight range … the choice between different types and combinations of materials and components or different types of mattresses are more of a preference and a budget choice than a “better/worse” choice (see this article). The best way to know which types of materials or mattresses you tend to prefer in general terms will be based on your own local testing or your own personal experience.

Each mattress category can also include hundreds of different mattresses with a very wide range of different designs, different “feels”, different characteristics, and different firmness levels. Every individual layer and component in a mattress (including the cover and any quilting material) will affect the feel and response of every other layer and component both above and below it and the mattress “as a whole” so each mattress category will generally include some mattresses that have a design that will be a good “match” for you in terms of “comfort”, firmness, and PPP and others that use the same type of materials and components and are in the same category and may be just as durable but have a different design or firmness level that may be completely unsuitable for you to sleep on … even if it uses the same general type of materials and components.

Latex and memory foam are very different materials with very different properties but again the choice between them is more of a preference and budget choice than a “better/worse” choice. There is more about some of the differences between memory foam and latex in post #2 here. Some people tend to prefer the faster response and more resilient and “on the mattress” feel of latex and some prefer the slower response and more “in the mattress” feel of memory foam but again the best way to know which type of materials or mattresses you tend to prefer in general terms would be based on your own careful testing and/or your own personal experience with each material in a range of different firmness levels.

If you would like to do some local testing if for no other reason than to get a general sense of the types of materials and mattresses you tend to prefer (regardless of whether you end up purchasing locally or online) then if you let me know your city or zip code I’d be happy to let you know about any of the better options or possibilities I’m aware of in your area.


Thanks, Phoenix. I’m in San Francisco, although I tend to head down south often.

While I understand there’s no standard for “firmness”, I do think that if the majority of complaints come from users saying it’s “hard as a rock” or “like sleeping on concrete”, then it would at least “feel” firm to me. But just for full disclosure, I used to like to sleep on very hard surfaces and am just making the switch now that my body is starting to ache in some areas…maybe it’s age :unsure:

Hi sliceoflife,

Subject to confirming that any retailer or manufacturer on the list is completely transparent (see this article) and to making sure that any mattress you are considering meets your specific criteria and the quality/value guidelines here … the better options or possibilities I’m aware of in and around the San Francisco/San Rafael/Oakland/Berkeley areas are listed in post #2 here.

Going south the list for the San Jose/Santa Cruz/Salinas/Monterey areas is in post #2 here and for the Bakersfield area is in post #2 here and for the San Luis Obispo area is in post #2 here and for the Santa Barbara/Ventura area is in post #2 here and for the Greater Los Angeles region is in post #2 here.

Hopefully that will cover most of the areas you go through when you travel south but there are certainly some very good options available in the SF area.