Memory foam density explained: 40, 50, 60 kg/m³ and what it means for you
If you only learn one number about a mattress, make it foam density. It predicts how long the comfort layer will last better than the brand, the price or the spring count, and it is almost never on the box. Here is what it means, what the ranges are, and how to get it out of a seller.
What density is
Density is the weight of a cubic metre of the foam, in kilograms. A 50 kg/m³ memory foam weighs 50 kg per cubic metre. It is a measure of how much material is in the foam versus how much air. More material means the cell walls are thicker, which means the foam recovers its shape for longer before it fatigues.
It is not the same as firmness. Firmness (hardness) is measured separately, as the force needed to compress the foam, and a high-density foam can be soft or firm. Density is about how long it lasts; hardness is about how it feels.
The ranges
| Foam | Density | What it is used for | Typical lifespan of the layer |
|---|---|---|---|
| Standard polyurethane (PU) | 20 to 30 kg/m³ | Cheap quilting, packaging, sofa cushions | Two to four years before it goes soft |
| Reflex or "high resilience" foam | 30 to 40 kg/m³ | Base layers, support cores | Eight to ten years as a base |
| CMHR (combustion-modified high resilience) | 35 to 50 kg/m³ | Comfort and transition layers in UK-made mattresses | Eight to ten years |
| Memory foam, low density | 40 to 50 kg/m³ | Toppers, thin comfort layers, budget mattresses | Three to five years before body impressions |
| Memory foam, medium density | 50 to 65 kg/m³ | Comfort layers in good mattresses | Seven to ten years |
| Memory foam, high density | 70 to 85 kg/m³ | Premium comfort layers, the "slow sink" feel | Ten years plus, but warm and heavy |
| Talalay latex | 65 to 85 kg/m³ | Cool, springy comfort layers | Ten to fifteen years |
Lifespans are rough and depend on your weight and the base. But the pattern holds: below 50 kg/m³, memory foam takes a body impression within a few years. Above 60, it does not.
Why a £299 mattress goes soft
Two mattresses from the same factory can look identical on the box: "pocket sprung, memory foam top, 25 cm deep". One is £299, one is £599. The difference is almost always in the foams. The cheap one uses 40 kg memory foam over 25 kg PU; the dear one uses 55 or 60 kg memory foam over 40 kg CMHR. Same springs, same cover, twice the foam cost, and a mattress that is still level in year eight instead of dished by year three.
You cannot tell by lying on them in a showroom. Both feel fine new. Density is the only way to know.
Hardness
Alongside density you may see a hardness figure, usually in newtons (N) or as an ILD/IFD number. It is the force needed to compress a sample by a set amount. For memory foam, roughly:
- Under 100 N: very soft, the deep "sink-in" feel
- 100 to 150 N: soft to medium, most comfort layers
- 150 N and above: firm memory foam, unusual on top
A "plush" memory foam is low hardness at decent density, like a 50 kg/m³ foam at 80 N. A cheap soft foam is low hardness because it is low density, which is a different thing and does not last.
Memory foam versus the alternatives
Memory foam is worth having where its pressure relief matters, under your shoulder and hip when you sleep on your side, and where it does not touch you if you sleep hot. It is not the right top layer for hot sleepers, and it is not the right material for a base. A good build uses it in the middle: a breathable layer on top, memory foam for pressure relief, CMHR to transition, springs or reflex foam for support.
How to get the number
Ask, in writing: "What is the density, in kg/m³, of each foam layer, and the hardness in newtons of the memory foam?" A maker who specifies their foams will answer in a sentence. A reseller who does not know will say "high density" or "premium memory foam", which are not numbers. If you cannot get a figure, assume it is under 50.
How we do it
Every Chapter product page lists each foam layer with its density, because that is what the factory works to and what a reviewer would measure. Our memory foam is 50 kg/m³ on the entry build, 55 to 60 on the everyday and support builds, and 65 in the lower comfort layer of The Hotel Bed, where a low-hardness 50 kg plush layer sits under natural fibres for the sink-in without the heat. Base foams are 33 to 35 kg/m³ reflex, 40 on Firm tension. See the full spec on any chapter.
The short version
- Density (kg/m³) predicts lifespan; hardness (N) describes feel.
- Memory foam under 50 kg/m³ takes a body impression in a few years.
- Ask for the number per layer, in writing. "High density" is not a number.
- Memory foam belongs in the middle of a good build, not on top, and never as the base.
What BS 7177 on a mattress label means, and what "low hazard" is
The fire-safety standard every UK mattress has to meet, in plain words: the four hazard levels, which one your bedroom needs, what the tests involve, and how makers pass it with and without fibreglass.
What is CMHR foam? The support foam in UK mattresses, explained
CMHR appears on British spec sheets and almost nowhere else. What the letters mean, what the foam does under the comfort layer, what densities are good, and how it differs from memory foam and reflex foam.
How deep should a mattress be? 20, 25, 30 cm and what the depth is made of
Depth is quoted everywhere and explains almost nothing on its own. What a good 22 cm mattress has that a bad 30 cm one does not, the depth each type of bed needs, and the sheets problem nobody mentions.