The Differences and Relationship Between Foam’s Density, Weight, and Firmness

If you were to tell a stranger that foam has density, weight, and firmness characteristics, he or she would likely understand, given how common the terms are. However, one of the most confusing things about foam is the relationship between these characteristics. On the surface, it would seem the density or weight of a material would allow you to draw a correlation about its firmness, and vice versa. In general, this is often true, but when applied to foam products, density and firmness are independent values for determining a foam’s qualities.

2.8 Pound Density HD36 Foam

2.8LB Density HD36 Foam

It would be accurate to say density is a foam characteristic that is “over-applied,” rather than one that is misunderstood. The density of foam means the same thing as any other application of the term; the quantity or mass of a material per a measurable size or volume. This pertains to all varieties of foam, including expanded polystyrene (EPS), polyethylene, polyurethane foam, and others. How density is measured varies across materials though, and in the case of foam, density is found by weighing a 12″ x 12″ x 12″ block of the material. If a product has a 3LB density, that means its 12″ x 12″ x 12″ block weighed 3LB. And while it’s vital to understand that density does not  pertain to the firmness of a foam product, it does correlate to the quality and longevity of a product.

Many conventional foams have a density between 1LB and 3LB. However, the densest materials can be as much as 10 or 15LB. High-density foam, Like The Foam Factory’s 2.8LB density HD36-HQ foam, is optimal for uses that receive heavy or daily use like couch cushions, bedding, or automobile seating. Lower density foam is excellent for occasional-use products like shipping foam, crafts, or guest room mattress toppers.

Density is also sometimes referred to as weight, which is a more literal translation of the characteristic given the testing process. But because of this, it’s always important to specify whether you want to know a product’s overall weight, or its density weight. Consider a 6-inch thick, conventional foam queen mattress with a 2.8LB density. The material weight is correctly stated as 2.8LB, since that’s its density. However, the overall weight of the mattress would be about 46LB. That’s about 43LB worth of reasons to make sure you clarify which value you need to know, since both can be technically correct.

1.4 Pound Density Filter Foam

1.4LB Density Filter Foam

Firmness meanwhile, interprets the feel of foam and how it yields to weight and pressure. Its measurement is called Indentation Load Deflection (ILD) (also known as Indentation Force Deflection/IFD), found by mechanical performance testing.  A foam sample measuring 15″ by 15″ by 4″ is used and the force in pounds that it takes a 50 square inch circular indenter to compress the material 1″ (25 percent of its thickness) is recorded. If the sample requires 36LB of pressure to indent it 1″, its ILD is 36. It is also important that the test material meets the standardized dimensions, as different thicknesses of the same material can support weight differently. A hard foam material will require greater force to reach 25 percent compression, and a softer material will require less. Most common materials have ILD values from 8 to 70, with some materials as high as 120 to 150. A low ILD example would be The Foam Factory’s 12ILD Super Soft Foam, while their Rebond Foam is very firm at 70ILD.

Firmness testing is done to help illustrate how a material will bear weight in end-use applications. It is important to interpret firmness values as an explanation of a material’s physical feel rather than its quality, which is reflected by its density. Because of the numerous structural and chemical makeups of foam, some foam sheets with higher densities can even have a lower ILD than foams with lower densities. For this reason, the two values should be looked at independently and used to help find a product that matches your preferences.

Understanding what these characteristics do and do not tell you about a foam material is very important for selecting the perfect product for an application. By understanding the values of these measurements, you can have a better idea of what to expect from a product and make a more educated purchase.

For questions or more information about foam density, weight, and firmness, contact The Foam Factory here.

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181 Responses to “The Differences and Relationship Between Foam’s Density, Weight, and Firmness”

  • Peter Appel says:

    Is a 12-13 I.L.D. paired with a 5.34 lb. density foam good for a memory foam topper dssigned to “soften up” a mattress that is too firm? Thanks, Peter

    • Foam Factory says:


      The simple answer to your question is yes. A 12-13 ILD foam is on the softer side of the firmness spectrum, so it will soften a firm mattress.

      Now, the actual firmness of the mattress it will be going on does play a role in the feel of the topper. If you have a foam mattress with a hypothetical ILD of 50, you can go directly by the numbers and know a 12-13 ILD topper will be softer. Lower ILD is softer feel, higher ILD is firmer. With conventional innerspring mattresses, feel is a bit more subjective, but if you have a mattress that is genuinely too firm, a 12-13 ILD topper should soften it. Also, the density of foam reflects the quality and durability of the material more than the feel. In your case, with a 5.34 pound density, you’re looking at a very high-quality topper.

      Lastly, the great thing about visco-elastic memory foam for people who want a softer sleeping surface is that it is activated by the user’s body heat. This means the most cushion is offered in the places you need it most: high-pressure areas.

      If you have any other questions, don’t hesitate to ask, whether it’s here or through phone or e-mail via our Contact Us page!

  • Evelyn Hill says:

    Hi, we need to firm up out IKEA mattress, manufactured at 70 kg/cubic meter, or 4.4 #/cu ft. I do not know the ILD maybe 11 – 12. This seems to be a very soft mattress at 7″ deep, single layer foam. We are considering adding 2″ – 4″ of more dense foam in the range of 5.2# to 5.7#. Should this be installed on top of, or below the mattress?

    Thank you for your help, we are not in the trade, but have been attempting to educate ourselves in the world of ILD, IFD manufacture.

    Evelyn Hill & James Tuthill

    • Foam Factory says:

      Evelyn and James,

      This is an excellent question, and it’s quite apparent you’ve done research to try and get some insight on your situation which should be applauded.

      Unfortunately, there isn’t a simple fix for the situation you’re in. It’s easy to soften a firm mattress, but you can’t really make a soft mattress firmer, at least to the degree most people would want. A 4.4LB density mattress is a good quality material, but you are spot-on that an 11 to 12ILD is a very soft foam, especially for a mattress. At 7”, this softness becomes even more pronounced, as you can sink too deeply into the bed. Adding a firmer layer over a softer layer will be unsupportive, as the softer base will cause the firmer top layer to bow, with the soft foam yielding to the weight. You are correct that a higher ILD foam (35-50ILD) would be firmer in terms of feel, but the performance would be greatly diminished by the soft base foam if you were to place it on top. Placing firmer foam beneath soft foam won’t have an effect on the feel unless you’re bottoming out though all 7” of the soft foam – and if this is the case, the mattress needs to be replaced anyway, as a foam that thick should not be that soft for supportive, comfortable sleep.

      If it’s your intention to turn a very soft mattress into a firmer one, we’re sorry to say that our best suggestion is to replace your mattress with something that fits your needs better. If the mattress was layered and built from multiple sheets adding up to 7” you would have some maneuverability, but in a single slab, there aren’t many options.

      If you have any more questions about ILD, softness, support, and mattresses, we would be more than happy to help you get the comfort you’re looking for! Feel free to follow-up with additional questions or comments here, or you can contact us directly through one of the channels on our Contact Us page.

      -Foam Factory

  • Kate Kleiman says:

    We have a sleeper sofa with inferior cushions and they are slipping and sagging after only 18 months. I’m looking for replacements, and I need to know what density and ILD I need to support two weighty people over the long haul (200 & 250 lbs.) Can you explain the relationship between density, ILD and support for higher weights?

    • Foam Factory says:

      Hi Kate,

      Sorry to hear those cushions are already giving out on you, but the good news is, we can definitely help! Thank you for commenting as well; this is a great question and one we get asked pretty frequently, so hopefully this gives you, as well as others who have the same problem, some insight.

      The biggest thing to understand about density and ILD, and something you’re probably already aware of, is that they’re independent of each other. But in concert, they form the overall experience you get out of a cushion. In a situation where you want a longer life out of a foam insert, with adequate support for the requirements you’ve specified, your number one priority should be high-density foam.

      Since foam density is calculated uniformly – the weight of a cubic block of the material – a high-density foam contains more performance product (physical foam material vs. air pockets) than lower density foam. This is indicative of greater overall quality, and makes for a more resilient product that is stronger, can handle greater levels of use, and will retain its comfort qualities longer. Even with hours and hours of heavy use every day (constant getting up and getting down, falling into seats, kneeling on cushions, etc.) 18 months is a very short timeframe for cushions to wear out, which tells me your current foam is lower density, even if they started out comfortable. For the long-term durability you’re after, a couch cushion should be between 2LB and 3LB density, for the best blend of benefits and value.

      ILD, on the other hand, measures firmness. The lower the ILD, the softer the foam, and the higher the ILD, the firmer the foam – no tricks there! But the thing that sometimes confuses people is that feel (soft or firm) has no bearing on the quality or durability (density) of the material. You can have a high-density cushion with a very low ILD, which means it will be a very soft cushion but will maintain that same degree of softness for years. For some people, that’s exactly what they want. In your situation, where you’re looking for support, higher ILD is desirable, because firmer foam doesn’t create that softer “sinking” feeling. Higher ILD foam is still soft and comfortable, but you will feel more stable and rest higher in the cushion, which also makes it easier to get in and out of. It is also possible to have a high ILD foam with a low density, which will start off firm, but quickly lose its supportiveness, causing it to become soft, spongy, and saggy, which sounds like what happened to your current cushions.

      In your situation where you want something durable that will comfortably hold up for years while being very supportive, we would suggest our Lux-HQ foam. It features a density of 2.8LB and an ILD of 50LB, which is very firm in the world of comfort and support products. This is our most durable cushioning foam, as well as the firmest, and typically lasts a minimum of a decade, and up to 18 years in some cases. Our HD36-HQ is also a 2.8LB density material with the same lifespan, but has a softer blend of comfort and support, with a 35ILD.

      Hopefully you found this information useful and feel a little better equipped making your cushion replacement decision. If you have any other questions or comments, don’t hesitate to ask, whether it’s in another blog comment, through our Contact Page, where you can find our phone number and e-mail address, or even tweeting at us on Twitter (@FoamFactoryInc)! Thanks again for the great question, Kate!

      – Foam Factory

  • Gene spain says:

    Your site is very helpful. Looking for a very firm foam mattress. Saw your hd36hq vs. LHQ 2. Are they latex or some other foam? Open or cloed cell or what do I want? We sleep on one that is so firm it almost does not compress at all and we love it but don’t know it’s numbers or how to duplicate it. What ILD d I want? TNX.

    • Foam Factory says:

      Hi Gene,

      Those are some great questions. First off, both our HD36-HQ and Lux-HQ are polyurethane foams. We do carry all-natural Talalay latex products, but each is marked as such. Polyurethane foam is the squishy, soft foam you’re likely used to in couch cushions, though there are many different varieties.

      For mattresses, you always want to select open-cell foam, whether you want the softest or firmest surface. Closed-cell foam is simply too firm for a comfortable, pressure-free mattress.

      In regard to our HD36-HQ and Lux-HQ foams, one is definitely firmer than the other. The HD36 carries an ILD value of 35, which is neither too-firm or too-soft; it is a middle-ground blend of comfort and support. Lux-HQ meanwhile has an ILD of 50, which is very firm, and what you’ll want for the mattress you’ve described.

      Lux-HQ’s 50ILD is a very firm material in the big-picture, and the firmest mattress material we sell. The higher the ILD number, the firmer the material, so 50 is the range you want to shoot for. For perspective, rebonded carpet padding, an ultra-firm product not generally suited for bedding, has an ILD in the 70s. To compare the firmness values of our other foam products, you can find their ILDs on their individual Data Sheets.

      I’ve passed your message on to our Sales Team who should be getting back to you soon with more information. In the meanwhile, feel free to Contact Us with any other questions you may have!

      -Foam Factory

  • OTmom says:

    How much does your 2 inch 4 lbs. King sized memory foam topper weigh? Just the topper and not the box or wrapping. I notice that some sites will advertise a 4 lbs. topper in King size, but the total weight is 22 lbs. That would be the weight for a 3 lbs. I already have that and would like to try a 4 lbs..
    Also, what will the 2 inch 4 lbs. topper feel like vs a 3 lbs? Will it be firmer or a little more like cookie dough? How supportive is it compared to a 3 lbs? Will it hold on longer (not flatten out?)

    Thank you!

    • Foam Factory says:


      Thanks for commenting!

      To answer your first question, our King-Size, 2″ thick 4LB ViscoPLUSH memory foam topper will weigh around 28LB total – foam only. You are also correct that a 2″ thick, King-size 3LB density memory foam topper should weigh around 21/22LB.

      As far as firmness and support, all three of our memory foam densities (3LB, 4LB, 5LB) have similar firmness values, around 14ILD per our data sheets. This test is usually applied to conventional foam however, and the temperature sensitivity of memory foam is a bit of an extenuating circumstance. That said, they are all about the same feel, which is based off the softening around your body’s contours.

      The biggest difference between densities in memory foam is the durability factor. As density increases, so does the amount of foam performance material vs air in a product, meaning there is more material that takes longer to break down. 4LB is longer-lasting than 3LB and 5LB is longer lasting than 4LB.

      Hopefully this helps, and if you have any other questions, don’t hesitate to ask!

      -Foam Factory

  • Adan M. says:

    This webpage is amazing and very helpful. Very much appreciate. I do have one question, if it makes sense. How does firmness and weight correlate (if there is a correlation)?

    • Foam Factory says:

      Hi Adan,

      Since weight is how you deduce foam’s density, you can use weight and density interchangeably here. As density and firmness do not have any consistent correlation, neither will weight and firmness. You can have very firm, low-density foam (light), very soft, high-density foam (heavier), or anything in between. I’ve passed your message on to our Customer Service Team who will follow-up with you in case you have any other questions. Have a great day!

      -Foam Factory

  • Frank G. says:

    Which one would you recommend for maximun durability as well as soft confort for a very heavy 412 lbs guy? I once bought a foam mattress that was supposedly among the firmest, and during a long long time I wished I had bought a much softer one…

    My bed size is 54″ x 75″.

    My choice would be between the HD36 Foam HQ and Lux Foam HQ.

  • c. harmon says:

    Your website is outstanding and the Q&A informative; I hope you are rewarded with high volume sales! My questions have been answered except for one:

    Over a few years will the foam dimensions shrink, expand, or remain stable? I am considering two 34 x 74″ slabs about 6″ thick of either HQ HD36 or HQ Lux. The dimension leaves about 1″ around all perimeters of the steel bed frame and allows for thick bedding. This is the maximum space I desire so if the foam shrinks the 1″ space will increase beyond what I desire.

    The frame includes no-sag springs (contract style dormitory bed), upon where the mattress will sit directly (no box foundation)

  • Scott says:

    I’m looking for 1.5″ closed cell polythene foam to serve as mats for Brazilian Jiu Jitsu – what would be the best density to use – we have a lot of 300lb guys. We will be covering the foam with 18oz vinyl.

  • Dudley Brown says:

    Our touring caravan seats lose their support after sitting or sleeping on the after a short while. If we were to refill them we wouldn’t know which foam filling to ask for.
    The caravan is a Lunar Lexon 575 EB made in 2008. The seat cushions in question are approx. 60″ long x 24″ x 6″
    We live in East Cheshire and the van is on site in N. Wales

  • robyn matlock says:

    I purchased sofa where the back flips down into a sofa bed. The entire sofa is made of a built in inner springs mattress. There are no removable cushions. However it is just too firm, even to sit on as a sofa. As a bed I could put on a memory foam topper, but how do I soften the sofa seat when used as a sofa? I decided to create a two inch topper pad, (more than 2″ would make the sofa seat too tall) but what is the best foam to add softness for sitting pressure: high density 4 pound memory foam or regular foam or polyester filler or ?????

    I found a slip cover website that could custom make the pad cover, but now I need to find the foam

    Robyn Matlock

  • Miranda says:


    I am 1.65 cm and weigh 135 Lb. I have fibromayalgia and currently sleep on a Cotton foam mattresses which is too firm for me. I have used a 2LB density Memory foam 4″ topper on my futon mattress, that was too soft and uncomfortable, and hot, I was sinking into the mattress. So I am planning to get an Air bed and use a Latex topper on that. However I am not able to decide on the firmness. If to get a 3″ Soft or Medium. will soft be too soft that I will sink into that?

  • Pamela Beeth says:

    I am a female 5’4″ and weigh 200 lbs. My arthritis has continued to worsen over my 76 years and now even my Tempupedic mattress is a little too hard for me to sleep on my side because of the pressure on my shoulder and hip. I hope you can suggest a soft but supportive topper for my twin sized bed.
    Thanks for the information on this site and your attention to my question.

  • Christin says:

    Hi, I’m making an upholstered headboard and am thinking about using a 1″ mattress topper instead of high density foam, which is much more expensive. Is there any reason this would not work as well as high density foam?
    Thank you!

    • Foam Factory says:

      When you are dealing with a headboard, all you need is a medium conventional foam. A high density is not really needed, only a medium to medium-firm foam is needed. When we do headboards in-house, we actually use our economical Poly Foam. Anything more in terms of density or firmness is unnecessary.

  • mike lasare says:

    Can you explain how the “egg crate” LUX foam would different from the solid LUX foam feel as a matress topper?

  • Great web site. My question is regarding cushions in living room chairs we are going to be re-upholstering.
    The chairs are 25 years old and the foam needs to be replaced. I am wanting a firm foam that, maybe not lasting another 25 years, but won’t sink when sitting in the chair making it harder to get up and out of, bad knees and all.
    With those requirements in mind, I think it would be difficult to find the right foam at the local fabric store.
    Thanks Again, Amelia
    Thanks. Again, site so very helpful.

    • Foam Factory says:

      If you want a firm foam, we would suggest our Lux foam. If you wanted a medium foam, I would suggest our HD36 foam. The regular grade has a life of about 7 years, HQ grade foam lasts about 15 years.

  • Real Mathieu says:

    What density would you recommend to reupholter dining room chairs that currently have 2 inch foam seats that are “tired”

  • Stevan says:

    Hi, I’m reupholstering my computer-office chair. I decided that overall thickness is going to be 2 inch, which will consist of hard bottom layer end soft top. I already bought hard layer. It is has 25% ILD of 33. What is your recommendation for a soft layer hardness (memory foam is out of the question) and in what proportion their thickness should be for a comfortable 8h of siting? Thanks

  • Bill says:

    What would you generally recommend for rebuilding a Harley Davidson touring model motorcycle seat. I would likely be cutting out approx 2″ deep section of the seat area and then blending the suggested foam.

  • Anna says:

    I have a new innerspring (low quality) queen mattress that is already showing a tendency to sag in the middle and is not comfortable. I use a 2″ natural latex topper. I am looking for a layer between the mattress and latex topper that will provide overall stability with a comfortable feel for a 5’8″ 145lb. side sleeper. I am looking at a 3″ Lux HQ or HD36 HQ. What do you recommend?

    • Foam Factory says:

      Unfortunately when the base layer or foundation starts to show wear, the only solution we suggest is a replacement. There isn’t much that will prop up a sagging mattresses. Sorry about that.

  • Tom says:

    We are looking to replace our sofa cushions but are unsure to go with the 2.8 lb denisty,35 ILD or the 2.8 lb, 50 ILD. We like the cushions firm but are not sure if the 50 ILD will be too firm. I weigh about 175 lbs and my wife is about 120 lbs. Can you you advise?

  • Larry Douglas says:

    I love your site and what you have to offer. We purchased a 38-ft sailboat a couple of years ago. This year I decided to a heavy detail cleaning bow to stern, inside and out. Treating for mildew and mold. The boat is spotless now. But the cushion for the cabin and berthing had a smell. We had them professionally cleaned, but that did little. So I pulled the covers and discovered mold. I cleaned (large tub of water) and treated. I am having a terrible time getting them to dry. After a couple of 80 degree weather they are almost dry.

    The almost is a major concern. I wondering will they ever dry completely or should I just order new foam. If so should it be the HD36 or Lux for the cabin seating cushions.

    Plus: what am I looking at for delivery time (Freeland, Wa), days, weeks or months. The reason I ask is we are on a schedule to head out on a long trip.

    • Foam Factory says:

      Foam that has mildew should be definitely be replaced. Normally if the foam is wet, but has no mildew, placing a fan in front of the foam would help it dry faster. Both HD36 foam or Lux foam would work well. The foam should be covered in plastic to help make it water resistant. We also have dryfast foam available that can be used for boat applications. In either case, the foam should be dried seasonally or every 3 months to prevent mildew.

  • Meaghan says:

    I am looking for a foam that will support up to 300lbs when stacked in 3 or 4 thick layers. Such as the product from Fatboy Baboesjka stackable pillows. I would like to do my own DIY version. What would work best for this project?

  • Jan Levy says:

    I have a reclining sectional that is showing deep body impressions after 10 months of use.The the seat cushions and back cushions are different thickness. The current foam is a Diminished Density foam( for enhanced customized comfort.The retailer claimed diminished density meant the foam gets softer the closer to the top and it is still very durable. What foam would you recommend that I use when I replace the foam?

    Thanking you in advance for your consideration.

    Jan Levy

  • Roxana says:

    Hi,im looking for a mattress topper for a full bed. I am a female 5’2″ and weigh 200 lbs. and I have arthritis in my neck and in the last vertebras which cause me pain in my low back. I hope you can suggest me the best option. I am not asking for a full mattress because my husband is a truck driver and I travel with him, so the truck already has a mattress. I was thinking about a memory foam 3-inch 5-pound,but I’m not sure. I would like to know your opinion.
    Thank you for your advice 🙂


    • Foam Factory says:

      If the mattress is soft or worn out, then you may need to replace the mattress and place a layer of memory foam on top. If the mattress is medium to firm, then yes we would suggest our memory foam for your application. In your case, we would suggest 4″ of our 5lb density memory foam topper.

  • Denise Novak says:

    WE have a 2010 Prius with the most uncomfortable seats ever. I have just gone and to an auto Upholster and had the seats redone and yet it is still too firm. What can you suggest to make the foam less firm or stiff. It hurts to sit on it. I have put some weights on the top of the seats but all I have is about 40 lbs and we weight more then that. But I’m leaving them all whenever I’m not in the car.

  • Lisa says:

    Hi, we have a sailboat and need to replace the mattress sleep on. It must be a 3 inch thick mattress and will be inserted into the mattress vinyl cover the current mattress is in. It will be used on a solid fiberglass base with no give. I have lower back stiffness and need support. The problem with the current mattress is due to age, I sink to the base through the mattress while sleeping. We are planning to add a memory foam mattress pad for comfort over the vinyl surface. We are thinking of getting the most firm mattress available in 3 inches, or should we go with the next level down? We are both side sleepers. Which would you recommend?

  • Jasmine says:

    What foam do you recommend for a side sleeper that has chronic hip pain. I have been told 33 HD, but I am only seeing 36 HD. I do not like memory foam. My boy friend likes a firm mattress but I need softness for my hips. I would like the mattress to be the same width and length as my queen bed I have now so I gather I will need to have 2 pieces glued together?
    You help is appreciated. Thanks.

    • Foam Factory says:

      Yes, we would suggest our HD36 foams for your bedding needs. Our HD36 is very close to a 33, even if you had a 36 ILD foam next to a 33ILD foam you would not be able to tell the difference. Yes, we can glue different types of foams together so you could have medium on one side and firm on the other side of your mattress. Please contact us for further suggestions and pricing.

  • susan richardson says:

    We have a pop up camper that we are planning to take on a long trip. The foam mattress it came with is not firm enough to provide a comfortable night sleep for any length of time. We weigh 165 and 116 lbs each. What type of foam should I look for to replace the standard foam we have?

  • Pastor Shalach says:

    Hi! We are trying to make a bunch of leather backpacks more comfortable. On back of each backpack is a pocket that reaches all the way down to the bottom seam. We would like to purchase some foam to put in said pocket to make it act as a barrier to the lower back. We have looked through your site but have no idea what to get.
    What would you recommend for us? Please share in laymens terms and a link to your product so we can order ASAP thank you !! Your site is awesome!

  • Susan says:

    Need to redo window seat cushions. Which of the following is the best foam 1) high density 2) extra high density 3) air lite high density and what is the difference

  • Anton says:


    Are you guys familier with Fastcap, or Kaizen brands of foam tool organizing systems? They are a pretty stiff, easy to cut foam. I have seen packing foams that are similar. I am trying to find a place to get foam similar to this without having to pay Kaizen’s prices.

  • GT Fletcher says:

    Hi, you have a great, informational website.

    I’m wondering what type and firmness of foam you would recommend for a weightlifting bench. I’m looking for something about 3-4″ thick and would deflect comfortably but minimally between the shoulders when one is bench pressing a large amount of weight, between 300-500 lbs.