Quick Answer: ILD (Indentation Load Deflection) measures how firm a foam feels. Foam density measures how durable it is. They are completely different numbers. A firm foam can be low quality. A soft foam can be highly durable. To evaluate any foam mattress properly, you need to know both: ILD tells you how it will feel on day one; density tells you whether it will still feel that way in year five.
In This Guide
Reading Time: 8 minutes
What ILD Measures and How It's Tested
ILD stands for Indentation Load Deflection. It is the standard measure of foam firmness used by mattress manufacturers, and it has a precise physical meaning: it is the number of pounds of force required to compress a foam sample by 25 percent of its thickness, measured over a circular area of 50 square inches.
The test is defined by ASTM D3574, an American standard widely adopted across North American mattress manufacturing. A foam sample is placed under a circular indenter plate. The plate presses down until the foam has compressed to 75 percent of its original height. The force reading at that point, in pounds, is the ILD number.
What this means in practice: a foam with an ILD of 14 requires only 14 pounds of force to compress that 25 percent. You will barely feel resistance when you lie on it. A foam with an ILD of 44 requires 44 pounds of force. It feels firm and pushes back noticeably. Most sleeping surfaces for adults fall somewhere between 14 and 44 ILD, depending on the layer's intended purpose.
One detail that trips people up: ILD is temperature-sensitive for memory foam specifically. Memory foam softens as it warms to body temperature. A memory foam layer might measure 20 ILD cold but feel more like 14 ILD after you have been lying on it for 20 minutes. This is part of what makes memory foam feel like it "hugs" you, and also part of why memory foam sleeps warmer than other materials.
ILD Firmness Scale (General Reference)
- 10-14 ILD: Ultra-soft. Comfort layers only. Not suitable as a support layer for most adults. Latex at this range feels like a cloud; poly foam at this range won't last.
- 15-21 ILD: Soft. Common in plush and pillow-top comfort layers. Suitable for lighter sleepers or as top-of-stack comfort material.
- 22-28 ILD: Medium. The most common range for all-foam sleeping surfaces. Works for side sleepers and combination sleepers who need pressure relief without bottoming out.
- 29-35 ILD: Medium-firm. Good for back sleepers and heavier side sleepers. Provides support without feeling rigid.
- 36-50 ILD: Firm to very firm. Preferred by stomach sleepers and those with significant back support needs. Can feel uncomfortable for side sleepers at pressure points.
- 50+ ILD: Extra firm. Used in support cores and industrial foam. Not typically found in comfort layers of consumer mattresses.
ILD Values: What the Numbers Mean for Latex vs Memory Foam
The same ILD number feels different in different foam types. A 25 ILD latex and a 25 ILD polyurethane foam are both "medium" by definition, but they feel different under your body. Latex is responsive, springing back when you move. Poly foam and memory foam are slower to respond. The ILD number tells you the resistance at 25 percent compression; it does not describe the material's feel throughout the full range of motion.
For latex specifically, the firmness range typically runs:
- Soft latex: 19-24 ILD
- Medium latex: 24-28 ILD
- Medium-firm latex: 29-34 ILD
- Firm latex: 34-40 ILD
These are somewhat lower numbers than poly foam for equivalent perceived firmness, because latex has a different stress-response curve. A 24 ILD latex does not feel the same as a 24 ILD polyurethane foam; the latex will feel more supportive and bouncy at equivalent compression forces.
Foam Density: The Durability Number
Density is a weight measurement, not a firmness measurement. It is calculated by taking a one-cubic-foot sample of foam and weighing it. A foam with a density of 1.5 pounds per cubic foot (lb/ft³) has 1.5 pounds of actual material in each cubic foot of foam. A foam with a density of 4 lb/ft³ has almost three times as much material in the same volume.
More material means more polymer chains to distribute stress. When you lie on foam, you are compressing those chains repeatedly, night after night. Lower-density foam has fewer chains to distribute that load, so the structure fatigues faster. The foam begins to soften and bottom out, which is what mattress owners describe as the mattress "going soft" or developing a body impression.
Higher density foam retains its original ILD longer because the greater material volume resists fatigue. This is why density is a durability indicator. A 1.2 lb/ft³ polyurethane foam might have a perfectly pleasant ILD when new, but it will likely degrade noticeably within three to four years. A 1.8 lb/ft³ foam of similar ILD will last significantly longer under the same conditions.
Foam Fatigue and Body Impressions
Research on polyurethane foam fatigue, including work conducted under ASTM D3574 protocols, consistently shows that lower-density foams lose a greater percentage of their original ILD after repeated compression cycling than higher-density foams. This is the mechanism behind mattress body impressions: the foam under your heaviest body zones (hips and shoulders) compresses more repetitions and fatigues faster, creating a permanent valley. This is also why mattress warranties in Canada often specify a 1-inch or 1.5-inch sagging threshold: below that, fatigue is considered normal wear. Understanding density helps you choose a mattress that stays above that threshold longer.
8 min read
Density Benchmarks by Foam Type
Polyurethane (Poly) Foam
Poly foam is the base material in most mattress comfort layers and support cores that are not memory foam or latex.
- Under 1.5 lb/ft³: Budget foam. Fine for guest mattresses or temporary use. Expect noticeable softening within three to four years with regular use.
- 1.5-1.8 lb/ft³: Standard quality. Appropriate for mid-range mattresses. Will last five to seven years with reasonable care.
- 1.8+ lb/ft³: High quality. Durable enough for primary mattresses used nightly. Often used in the support cores of better-quality mattresses.
Memory Foam
Memory foam uses different density thresholds because the viscoelastic structure is inherently more material-intensive.
- Under 3 lb/ft³: Budget memory foam. Softens relatively quickly and may not hold its ILD through a warranty period.
- 3-4 lb/ft³: Mid-range. The most common range in mid-priced memory foam mattresses. Reasonable durability for a five to eight year lifespan.
- 4-5 lb/ft³: High quality. Preferred for primary mattresses. Retains ILD significantly longer than lower-density alternatives.
- 5+ lb/ft³: Premium. Found in high-end medical-grade and specialty mattresses. Maximum durability.
Latex Foam
Natural latex is measured differently and tends to be inherently more durable than polyurethane foams at equivalent weight. Natural latex density typically ranges from 4 to 6 lb/ft³, but because the material itself has excellent elastic memory, it resists fatigue better than synthetic foams of similar density. Talalay process latex (used in our Restonic Revive Tiffany Rose at $1,995) tends to have a more open cell structure and better airflow than Dunlop process latex, though both are durable choices.
The Question to Ask In Any Canadian Mattress Store
When a salesperson describes a foam mattress to you, ask two questions: "What is the ILD of the comfort layer?" and "What is the density of the foam in pounds per cubic foot?" If they cannot answer both, that is worth noting. Reputable manufacturers publish these specifications. At Mattress Miracle in Brantford, Brad is comfortable walking through the construction of anything in our showroom. If a spec is not in the literature on hand, he can find it. Knowing these two numbers before signing anything protects you from spending $1,500 on a mattress that will need replacing in three years.
The Confusion That Costs Canadians Money
The single most common misconception we encounter is the assumption that firmness and durability are related. They are not. A high ILD foam is not necessarily a high-density foam. A mattress can be described as "firm and supportive" and still be made of low-density material that will compress in two years.
This matters because marketing language exploits the confusion. A mattress described as "firm orthopedic support" may have a high ILD comfort layer made of 1.2 lb/ft³ foam. It will feel appropriately firm when you lie on it in the store. After 18 months of nightly use, the body zone under your hips will have softened by 20 to 30 percent because the density was never sufficient to resist fatigue. The mattress did not break. It just wore out on schedule for its density class.
In our experience helping Brantford customers, this is one of the most reliable predictors of early mattress disappointment. The remedy is simple: ask for both numbers before buying.
How to Use ILD and Density When Mattress Shopping
A Practical Evaluation Checklist
- Determine your ILD target first: Based on your sleep position and weight, identify the ILD range that suits you. Side sleepers typically do well between 18 and 28 ILD for the comfort layer. Back sleepers usually prefer 28 to 36 ILD. Stomach sleepers often need 35+ ILD to keep the spine from bowing.
- Then evaluate the density: Once you have found a mattress in your ILD range, ask about density. For memory foam, you want 4 lb/ft³ or higher for a primary mattress. For poly foam base layers, look for 1.8 lb/ft³ or better.
- Consider the layer stack: A mattress has multiple layers. The comfort layer ILD determines how it feels initially. The support core density determines long-term structural integrity. Both matter.
- Factor in the warranty threshold: Canadian mattress warranties typically define premature sagging at 1 inch or 1.5 inches. A low-density mattress may technically stay above that threshold (in the warranty's measurement conditions) while still feeling significantly degraded. Density is a better predictor of real-world longevity than warranty language alone.
- Test in your actual sleep position: ILD is one number, but how a mattress responds under your specific body weight and at your specific pressure points is what actually matters for sleep quality. In-store testing for 10 to 15 minutes in your usual sleep position, combined with knowing the spec numbers, gives you the most complete picture.
Why This Matters in the Current Canadian Market
The mattress industry in Canada has expanded significantly with online-direct brands, and many of them are genuinely good products. However, some online brands are deliberately vague about foam specifications, describing mattresses in terms of "feel layers" and "support layers" without giving actual ILD or density figures. In our 40+ years at Mattress Miracle, we have seen the pattern: lower-quality foam specs tend to be the ones that are not published. When Brantford customers come to us after an online mattress has disappointed them, foam density is the first place we look. The solution for your next purchase is asking for the spec sheet before you buy, wherever you buy it.
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Popular picks at Mattress Miracle:
- Restonic ComfortCare Dalton and Albany
- Whitney double-sided bamboo mattress
- Somnia 3.0 neck support pillow
Or shop our mattress collection in our Brantford showroom.
Find Your Perfect Mattress at Mattress Miracle
We are a family-owned mattress store in Brantford, helping our community sleep better since 1997. Come try mattresses in person and get honest, no-pressure advice.
441 1/2 West Street, Brantford, Ontario
Call 519-770-0001Frequently Asked Questions
What is a good ILD for a side sleeper?
Side sleepers generally need a comfort layer between 18 and 28 ILD to allow the shoulder and hip to sink enough for spinal alignment without bottoming out. Lighter side sleepers (under 130 lbs) often prefer the lower end of this range (18-22 ILD). Heavier side sleepers (over 200 lbs) typically need the higher end (24-28 ILD) to avoid excessive sinkage.
Is a higher foam density always better?
Higher density is better for durability, but there is a practical ceiling. Memory foam above 5 lb/ft³ is very heavy and can make mattresses difficult to move or handle. Very high-density poly foam can also become less breathable. For most Canadian primary mattresses, the practical sweet spots are 4-5 lb/ft³ for memory foam and 1.8-2 lb/ft³ for poly foam comfort layers.
What is the difference between ILD and IFD?
ILD (Indentation Load Deflection) and IFD (Indentation Force Deflection) measure the same thing and the numbers are interchangeable. IFD is the technically precise term used in ASTM D3574 standard testing. ILD is the term more commonly used in consumer mattress marketing. When you see either number, you can treat it the same way.
Can I find out the ILD and density of a mattress at Mattress Miracle?
Yes. We can provide foam specifications for the mattresses we carry, including Restonic, Sleep In,. Brad at our Brantford showroom at 441 1/2 West Street is happy to pull spec sheets for comparison before you make a decision. Call (519) 770-0001 or stop in during our regular hours.
How does foam density affect mattress body impressions?
Low-density foam fatigues faster because there is less material to distribute repeated compression. The foam under your heaviest body zones compresses more times per night than other areas, so those zones fatigue first. This creates the visible body impression that looks like a valley where you sleep. Higher-density foam resists this fatigue longer because more polymer chains share the load over time.
Sources
- ASTM International. (2023). ASTM D3574: Standard Test Methods for Flexible Cellular Materials - Slab, Bonded, and Molded Urethane Foams. ASTM International. astm.org/d3574-23
- Jacobson, B.H., Boolani, A., & Smith, D.B. (2008). Changes in back pain, sleep quality, and perceived stress after introduction of new bedding systems. Journal of Chiropractic Medicine, 8(1), 1-8. doi.org/10.1016/j.jcme.2008.09.002
- Radwan, A., Fess, P., James, D., et al. (2015). Effect of different mattress designs on promoting sleep quality, pain reduction, and spinal alignment in adults with or without back pain. Sleep Health, 1(4), 257-267. doi.org/10.1016/j.sleh.2015.08.001
- Zhong, Y., Talapatra, B., Lu, B., & Turner, R. (2022). Characterizing the physical and mechanical properties of commercial polyurethane foam mattresses. Polymer Testing, 105, 107427. doi.org/10.1016/j.polymertesting.2021.107427
- Okamoto-Mizuno, K., & Mizuno, K. (2012). Effects of thermal environment on sleep and circadian rhythm. Journal of Physiological Anthropology, 31(1), 14. doi.org/10.1186/1880-6805-31-14
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