What Is ILD (Indentation Load Deflection)?
ILD is a defined foam-industry test: the force, in pounds, needed to compress a standard 15" x 15" x 4" foam sample by 25% of its thickness (one inch), using a 50-square-inch circular indenter. It's specified for polyurethane foam under ASTM D3574; latex is commonly tested under a related but distinct protocol. Higher ILD means firmer foam — but almost no mattress brand publishes it.
The one thing to know: ILD (Indentation Load Deflection) is a defined foam-industry test: the force, in pounds, needed to compress a standard 15"x15"x4" foam sample by 25% of its thickness using a 50-square-inch circular indenter (ASTM D3574 for polyurethane foam; latex is commonly tested under a related but distinct protocol). Higher ILD means firmer foam. The catch: almost no mattress brand publishes it, because unlike a self-reported '1-10' label, ILD is a verifiable number a buyer could use to compare brands directly — which is exactly why its absence is itself the story.
How the test works
| Test element | Spec |
|---|---|
| Sample size | 15 in x 15 in x 4 in |
| Indenter | 50 sq in circular plate |
| Compression measured | 25% of thickness (1 inch on a 4 in sample) |
| Result reported | Force in pounds required to reach that compression |
| Governing standard | ASTM D3574 (polyurethane foam); latex commonly uses a related but distinct protocol |
Why ILD isn't the whole story
Identical ILD numbers feel different depending on construction. Memory foam has a slow, viscoelastic recovery that deepens the sink-in feeling at a given ILD; latex responds quickly and feels more buoyant at a comparable ILD; a hybrid's coil support core adds bounce and can make a soft comfort layer feel more supportive than the same foam over an all-foam base. So ILD is only directly comparable within similar constructions — a 28-ILD latex layer and a 28-ILD memory-foam layer will not feel the same. A foam's ILD is a per-layer, per-material figure — it doesn't describe a finished mattress's overall feel, which also depends on comfort-layer thickness, support-core design, and construction type. See Body weight changes how firm a given ILD or firmness label actually feels, because a heavier body compresses further into any given foam before reaching equilibrium. A lighter sleeper (under ~130 lb) sits nearer the top of a comfort layer and feels its true surface firmness; a heavier sleeper (over ~230 lb) compresses through the comfort layer into the firmer support core beneath, so the same mattress can feel noticeably softer to them than the label suggests. This is why firmness guides that ignore body weight are incomplete.
Frequently Asked Questions
ILD stands for Indentation Load Deflection. It's a defined foam-industry test measuring the force, in pounds, required to compress a standard 15-inch by 15-inch by 4-inch foam sample by 25% of its thickness (one inch), using a 50-square-inch circular indenter. It's specified for polyurethane foam under ASTM D3574; latex is commonly tested under a related but distinct protocol, so latex and polyurethane ILD numbers aren't always directly comparable.
A higher ILD means firmer foam WITHIN a similar construction and testing method — not automatically a firmer whole mattress. The support core, comfort-layer thickness, and construction type (memory foam vs latex vs hybrid) all change how a given ILD number actually feels in the finished product.
Most consumer mattress brands market with a subjective 1-10 number or a Soft/Medium/Firm word instead of ILD. A minority of brands — often ones that also sell raw foam components — do publish per-layer ILD figures on their spec sheets. Non-disclosure is common enough that we treat it as the norm, not the exception.
Only cautiously, and only within similar constructions. A 28-ILD latex layer will not feel like a 28-ILD memory-foam layer, because the two materials respond to pressure differently. ILD is most useful for comparing layers of the SAME material type, or for comparing a single brand's own product lineup.