Foam Density, ILD and GSM Explained
Density tells you how long a foam will last. ILD tells you how firm it feels. They are unrelated, and a shop that quotes one when you asked about the other is either careless or hoping you will not notice.
Density (kg/m³)
How much material is packed into a cubic metre of foam. Nothing about firmness — purely about substance.
Density is the single best predictor of how long a comfort layer will last, because a foam fails by its cell walls collapsing, and denser foam has more of them. If you only get one number out of a retailer, make it this one.
- Under 30 kg/m³ — support core territory, or a cheap comfort layer that will soften within two or three years.
- 30–40 kg/m³ — a reasonable reflex support core.
- 45–55 kg/m³ — a memory comfort layer that should still be doing its job at eight years.
- Over 60 kg/m³ — very durable, very slow, warm.
ILD (Indentation Load Deflection)
The force in newtons needed to compress a foam sample by 25 per cent. This is firmness, and it is the number behind whatever a brand's 1–10 scale says.
Low ILD is soft, high is firm. It only compares meaningfully between foams of the same type — a soft latex and a soft memory foam with the same ILD do not feel alike, because they recover at different speeds.
GSM (grams per square metre)
Weight of a fabric or a fibre layer per square metre. Used for covers, quilting and wool or cotton fillings.
Higher GSM in a quilt panel means more fibre and a plusher surface. It says nothing about the mattress underneath, so a high GSM cover on a thin build is a way of making a cheap mattress feel expensive in a showroom for twenty seconds.
The numbers that actually get hidden
Ask about these and watch how quickly a retailer changes the subject:
- Comfort layer density. Not the core — the layer you lie on.
- Comfort layer depth. Two centimetres of excellent foam is worse than five of good foam.
- Spring gauge, if it is sprung. Count without gauge is half the story.
What we publish, and what we do not
Being straight about our own gaps: every product page here carries depth, firmness, spring count, layer breakdown, guarantee and fire certification. We do not currently publish foam density or ILD per layer, because those figures live on the factory floor and we would rather show nothing than a number we cannot stand behind. If you need them for a specific mattress, ask us and we will get them from production.
Common questions
Is high density the same as firm?
No. See above — that confusion is the reason this page exists.
Does a higher GSM cover make a mattress better?
It makes the surface plusher. It does not change the support.
What density should I look for?
For a memory comfort layer in a mattress you intend to keep a decade, 45–55 kg/m³.