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Understanding Foam Firmness and Compression Strength and Applying The Values To Your Needs

When evaluating a foam product for use in a comfort application, perhaps the most useful testing value for determining its usability is compression strength. This value rates a foam’s feel and firmness by testing how it yields to or supports applied weight. These values are the easiest way to categorize materials, making the selection of a foam type easier by narrowing the field and even helping make a purchase decision if the chance to feel the foam in person is not possible.

36 ILD Open-Cell Poly Foam
36 ILD Open-Cell Poly Foam

Compression strength is evaluated through a test called Indentation Load Deflection (ILD), also known as Indentation Force Deflection. This is a computer controlled performance evaluation, with standardization allowing all materials tested using this process to be measured against each other on a balanced plane. This sort of standardization is necessary in the foam industry, where there are hundreds of similar products and only minor differences can make one foam type suited for an application and make another the wrong choice.

There are multiple ways foam strength is tested, but the most prevalent is the 25 percent compression test. As the main method of testing, if a material is simply stated as having an ILD value of “Z,” that refers to its 25 percent compression test results. The most important aspect of this evaluation is ensuring that a proper sample size is tested. Samples must be 4 inches thick, 15 inches wide and 15 inches long. Maintaining this consistency across every test is important because foam materials can bear weight and pressure differently depending on thickness and size, even if the sample is cut from the same bulk material. Thinner sections of foam will yield more easily to weight while thicker sections will be more resilient. Additionally, testing products of different dimensions eliminates the test’s control and, in turn, the ability to evenly compare results from one material to another.

To conduct the test, a circular metal indenter applies downward force into the testing sample resting on a flat surface until the foam is compressed 1 inch (25 percent of the sample’s 4 inch height). The amount of pressure the indenter requires in pounds to compress the foam to its correct height is its ILD. If 50 pounds are required to compress the sample 25 percent, its ILD value is 50. On average, most ILD values fall on a spectrum from the low teens up to the 60s and 70s.

With foam’s ILD values directly correlated to the weight needed to compress it, customers can use those  values to interpret the feel of a tested material type. The lowest ILD values indicate the softest and most cushioning foams, as they require the fewest pounds to be compressed. These products are often used in a pressure relief system as seating pads, mattress toppers and chair backs where comfort is the exclusive goal. ILDs in the high-20s to the mid-40s generally mean the foam carries a balance of comfort and support. These materials can also be used for pressure relief cushions where a little extra support is desired. These foam types are firmer than the lower-ILD types but can still provide cushioning pressure relief. These materials are used predominantly as a pillow or in body wedges where the product needs to support weight while offering the most comfort. They are also utilized in mattresses, shredded foam filling or for orthopedic support. ILD values in the 50s and greater are the most supportive and firm foam materials. These are most commonly used in seating cushions that bear the full weight of a body centered on a smaller area, automobile seating and the firmest bedding materials.

Recognizing that ILD is not indicative of a material’s quality is an important distinction to understand. The spectrum of ILD values only interprets a material as hard foam or soft, not long-lasting or high-quality. To understand those foam characteristics, a product’s density should be examined. Along with ILD, Density can help you find the perfect foam for creating a better mattress, pad or cushion.

35 ILD Open-Cell Medical Foam

In addition to the standard 25 percent test, there are additional compression strength evaluations that provide more insight into a material. Frequently included along with the 25 percent ILD data is a 50 percent compression value, which is a measure of the pounds required to compress a foam sample half of its height. This is tested because compression is not necessarily linear, so you cannot simply double the 25 percent compression value to know how it responds to greater pressure. Many types of foam have similar 25 percent ILDs but very different 50 percent ILDs.

Yet another test is support factor, which may also be called sag factor or compression modulus. This value is a comparison ratio between the 25 percent compression value and a deeper 65 percent degree of compression. The 25 percent value is divided into the 65, typically resulting in a value from 1 to 3 that more accurately represents support capability than a single test. Because foam is going to be compressed by 65 percent more often in real-world use than 25 percent, this combination generates a performance value while still utilizing the 25 percent ILD. The greater the support factor value, the more supportive the material is.

While only one of the various factors to take into consideration while evaluating a material for use in an application, compression strength remains one of the most important. Firmness, feel and support are the characteristics foam users are going to be most sensitive to and the ones that dictate if they like the material they have selected or not. Knowing how to interpret ILD values can make selecting the right material much easier, which can save you time, effort and even money.

64 thoughts on “Understanding Foam Firmness and Compression Strength and Applying The Values To Your Needs”

  1. Would it be safe to say that if I needed polyurthane foam for a Physical theraphy table top that would support 200 pounds very well; it could be 2″ thick and have a 26 density and about 45 compression? Please advise what a piece of this 27″ x 76″ would cost.

  2. I need a 24″ x 84″ piece of foam for a bench seat. If the foam is to be 3″ deep, what density would be required so that and adult of 200lbs would not compress the foam more than 1.5″ when sitting on the bench?
    I could use 4″ thick if that helps.

  3. We have an older camper with a sofa which converts into a bed-queen size. The current 4″ foam is too hard for my husband who is 6’3″ and weighs 210. What foam would be good for his sleeping comfort and also for a sofa to support sitting people.

  4. I am in the planning of making a sectional. Both me and my boyfriend are bigger him being about 290. The cushions should 28 by 28 roughly. I was thinking about 4 or 5 inches thick. I am torn between your Lux foam HQ or HD36 foam. Keeping in mind that it will probably be slept or napped on

  5. I am making a large dog bed . Need to know what compression of supporting foam to use. I prefer 3in . I am also using memory foam for the top layer.

  6. I need a 1.2meter X 0.4Meter foam to withstand a load of 4tons at 50% compressibility. Can you suggest me what foam has to be used? I am planning to have 4″ foam so that final thick will come down to 2″ by the time applying the 4MT load. Please suggest the density as well as Foam Material.

  7. I sit on a small sofa all day every day for my work on a laptop. I prefer a hard and supportive foam, perhaps with a cushioned layer on top. What type of foam will hold up under such frequent use? About how many years of wear could I expect to get from it?

  8. Hi, I have back pain also had surgery. Last time I went for physical therapy and found that table very comfortable. I did not have any pain as long as I lay down on that table. what kind of foam they use. I need firm mattress 2″ of 1″. what do you recommend

  9. Would your hd-36 6 inch be too soft for multiple hour tv binges sitting on my couch cushions? I am 220 lb 5’10. After how many hours would the foam fully expand to gauge correct firmness?

  10. Hi. I’m looking for a compressed foam similar to a “yoga” block. But I need it on a large scale application. The yoga blocks are about 700/800mm of very dense foam and I need it in sheet format. Any suggestions. Thank you.

  11. We are thinking about replacing the seat cushions in our large sectional. We love the sectional, but the foam in the seats have already worn out and are mushy! We want a firm foam to sit on, so you don’t “sink” so much. I am a bigger guy: 6′-0″ 210 lbs. However my mother-in-law in slight: 5′-8″ and 110 lbs. What foam would you suggest that could support me – but no be to firm for my mother in law?

    I am worried Lux HQ might be too firm for her. Then again, perhaps I will sink in the HD36-HQ. Help!?!?!

  12. I have a screened in porch. I would like to replace me patio seat cushions. They don’t get rained on, but of course are subject to humidity and cool temps. Is HD36 or to use?

  13. How would 8 inches of HD36-HQ Foam, plus 4 inches high dnsity memory foam feel like? Would it be medium firm, I am told the memory foam is medium firm! iIs this the wrong combination.? I am building my own bed!

  14. I am making 4 Bar Height benches. The bench is 38x29x18. Looking for a 2″ foam to upholster the top of the bench. Which one will work the best to not feel the edge of the seat?

  15. We bought a couch from a major retailed and I love it but I sink (I weigh 155 lbs) in the 6 inch cushion down to the frame. I would really love a more firm cushion and I’m wondering how hard it is to get it and what it would cost?

  16. I need a piece of foam that is 13.25 inches thick, 20 inches long, and 19 inches wide that firmly supports weight up to 200lbs. It is to be used as a cushion at our home. Could you make such a piece?

  17. I would like to make a dog bed that can roll up as easily as possible for travel and is comfortable for a 50 lb dog. Would like it to be at least 2-3 inches if possible. Thank you!

  18. Hi! What foam should I use if It needs to be a 1″ sheet such that if I put a 3lb brick on top it sinks about 1/2″? How is this calculated, generally?

  19. Hi. I’m looking for an ultra-ultra-lightweight foam for a backpacking chair. It must be comfortable for a 200lb person to sit on (on a hard surface) for at least an hour, and a 12″ x 16″ x 3/4″ piece must weigh less than 1 once. Got anything that will work?

  20. I am needing a 1″ foam for western saddle seats that is medium firm and does not compress flat under the riders pelvic bone points. It needs to last for many years in a saddle that is used for hours a day. Is this possible?

  21. I have a Bassett leather sectional sofa and one of the modules gets the most wear. It has 2 types of foam and corrugated zig-zag suspension spring suspension. The lower foam layer is 5 inches and the loosely attached topper is 2 inches. I need to replace all the foam. Can you suggest a good foam combination?

  22. Hello:
    Am planning to reupholster 6 dining room chairs. We want a 3” cushion that will compress comfortably about halfway under a 170 lb. person over four hours of card playing. Cost is not a consideration. We value your advice.
    Sincerely,
    Larry Lapham

  23. I have a built in bench that is 82″ long, 21″ deep and 15 1/4″ high. I want a foam cushion that when compressed will be standard chair height of 18″. What foam would you suggest?
    Thanks in advance for your help!
    Sarah Stevensob

  24. I am restoring an antique Morris chair. The seat is supported by 8 way tied copper coil springs covered with coconut husk, burlap and finish material. I need to make a cushion that is 22″x21″ by 6″ thick that will compress about 30% when a 160 pound, average build person sits on it. I want the very highest quality, long lasting foam that will meet those requirements. What do you reccomend?

  25. I’m looking to modify a stock motorcycle seat on my Honda Fury cruiser. I’m about 200 lb with gear on. Basically I want to shave the stock foam away, do a cutout for a gel pad, then layer a 1″ thick pad of memory foam on top of everything and shape the seat back. What type and density of open cell foam would be recommended for this retrofit? Thanks in advance!

  26. We are building a banquette in our dining room. What foam would you suggest for a 250 lb. person that would compress about 2″? The foam will be 22″ x 75″

  27. Hi I’m building two motorcycle seats from scratch and wondering the best foam choice? One bike has rear suspension and the other does not and I don’t want the foam to compress after an hour or two of riding. It would also be 1/2” layers to make it easier to conform to the seat pan and 2” thick finished. Thanks!

  28. I’ve read all the motorcycle seat foam suggestions, still not my answer.
    I have tried buying used motorcycle seats, savaging their foam but in the long run, they failed.
    I’m refoaming a touring seat for **many** long hours and years in the saddle (I’m retired). What foam combo will best suit this? The top layer will be the near molded form of the persons butt, (similar to a tractors seat). The beneath layers will be of stock thickness(s) and replaceable as they deteriorate or required of a persons weight differences. Said base layers are the seat pan shape and edge cut to match top custom layer.
    I’ve already tried different densities of medical foam. They formed quickly but compressed to nothing after an hour. Neoprene is too stiff in any thickness tho it’s good for edge support.
    It seems I’m either at your mercy or find some mini ‘Z’ car seat springs and layer up from that. But alas, that’s even more complicated.
    Ah-hem… HELP!?

  29. I am in the process of converting an 2 existing futon mattress (twin) made of cotton layers into a tufted mattresses that can be stacked up and used as a daybed or opened up into a larger bed. I wanted to give the current futon more support and was thinking of using a foam insert sandwiched in the middle of the existing cotton. I would like the foam layer to be 3 inch and very firm. What would you suggest?

  30. Hello,
    I am looking for a foam type that would be able to roll/fold into a portable carrying bag. The foam needs to be able to be compressed, lightweight, but expand and be comfortable enough for a child weighing 40 pounds to lay on. What would you recommend?

  31. I need to make new cushions for our boat. Cushions will be partially exposed to weather (under a bimini top) so I plan to use Dry Fast foam. However, I would like a firmer cushion that lasts longer than Dry Fast, if possible, and am considering adding a 1″ layer of a closed-cell foam under a 2″ layer of Dry Fast. Do you think this will work? If so, which closed cell foam would you recommend? Thanks!

    1. It could work, as long as you drill holes, about 1″ in diameter, throughout the closed cell foam layer. The water needs somewhere to drain, having a hole about 8″ to 12″ apart should work.

      1. Thanks, very helpful. I’m considering using Minicel T-200 1″ for the closed cell foam as it appears to be similar to foam used on a friend’s boat. Would something else work better? You carry so very many closed cell foams; difficult to choose.

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