Foam Mattress Density Guide: What the Numbers Actually Mean
Foam density is the one number that separates a mattress that lasts a decade from one that sags in three years. It is also the number most brands do not publish. This guide explains what it means, what the right numbers are, and how to use it when you are shopping.
What foam density actually measures
Density is mass per unit of volume: kilograms per cubic metre (kg/m3). A block of foam that weighs 30 kg per cubic metre is denser than one that weighs 20 kg per cubic metre. The denser foam has more material packed into the same space. More material means more structural support, better durability, and better resistance to compression under body weight over time.
Density is not softness. It is not firmness. You can have a soft high-density foam and a firm low-density foam. Density and feel are separate properties. This is one of the most common points of confusion in mattress buying, and some marketing exploits it deliberately by using feel language to distract from density numbers.
The density numbers and what they mean in practice
Under 18 kg/m3
Very low density. Used in packaging, cheap furniture, and low-grade mattresses. Will compress permanently under body weight within months of regular use. Not suitable for a mattress you plan to sleep on regularly.
18 to 24 kg/m3
Budget mattress range. Found in the cheapest retail mattresses and the bottom of some brand ranges. Fine for occasional use (guest rooms, camping trips). Will show body impressions and loss of support within 2 to 4 years of nightly use.
25 to 29 kg/m3
Mid-range. Acceptable for shorter-term use or for sleepers under 65 kg. Will perform for 4 to 6 years for most sleepers. Starts to lose support noticeably after that.
30 to 35 kg/m3
The quality range for regular mattress use. This is where the SABS standard sits for bedding foam in South Africa. 30 kg/m3 is the sweet spot that performs across most adult body weights (roughly 65 to 120 kg) for 8 to 10 years of nightly use. Mr Mattress uses 30 kg/m3 SABS-approved foam across every model.
Above 35 kg/m3
Specialist and medical applications. Orthopaedic and hospital-grade mattresses. Significantly higher cost. Not necessary for most home sleepers. The performance at 30 to 32 kg/m3 is sufficient for the vast majority of residential use cases.
How density affects durability over time
Foam is a cellular structure. It is full of tiny cells of air held in place by the foam material. Body weight compresses those cells every night. Over time, cells break down and the foam does not fully spring back. That is body impression and loss of support.
Higher-density foam has more material per unit of volume, which means more cell walls, more structural integrity, and more resistance to breakdown under load. Low-density foam has fewer cell walls, which fail faster.
This is why a mattress at 20 kg/m3 might feel comfortable in month one and have a permanent groove in month eighteen. The material was never dense enough to hold up under years of body weight.
Density and body weight: the match that matters
| Body weight | Recommended minimum density | Expected lifespan |
|---|---|---|
| Under 60 kg | 25 kg/m3 | 6-8 years |
| 60 to 80 kg | 28 kg/m3 | 8-10 years |
| 80 to 100 kg | 30 kg/m3 | 8-10 years |
| 100 to 120 kg | 30 kg/m3 | 6-8 years |
| Over 120 kg | 32 kg/m3+ | Confirm with manufacturer |
These are estimates for nightly use by a single sleeper. Two sleepers sharing a mattress compresses it more, so density requirements move up a tier.
Memory foam density: a separate number
Memory foam is viscoelastic foam. It has its own density specification, separate from the base support foam underneath it. When a mattress has a memory foam comfort layer on top of a base foam, both have density values. Both matter.
For the memory foam layer: 45 to 55 kg/m3 is the standard for quality viscoelastic foam. Below that, the memory effect is minimal and the layer breaks down faster.
For the base foam under the memory layer: 30 kg/m3 minimum. This is the support structure. No matter how good the memory layer is, if the base foam is low-density, the mattress will sag.
Most brands do not publish both numbers. If a brand publishes only "contains memory foam" without density specs, ask for both: the memory foam layer density AND the base support foam density.
What brands publish and what they hide
A brand that is proud of their foam density publishes it in kg/m3 on their product pages. A brand that is not proud of it uses language like "high-density foam", "premium feel foam", "advanced comfort layers", or "engineered support zones". None of those phrases give you a number. None of them let you compare.
Mr Mattress publishes 30 kg/m3 SABS-approved high-density foam for every model. Sloom does not publish foam density in kg/m3. Cloud Nine does not publish a uniform density specification across the range (because they have a range from budget to premium, and the numbers vary).
This is not a criticism of foam quality across those brands. It is an observation about what information is available to you as a buyer before you make a decision.
SABS and what it means for South African buyers
The South African Bureau of Standards (SABS) publishes standards for foam used in furniture and bedding. SABS-approved foam has been tested against those standards for density, durability, and fire safety. The approval is not automatic: the foam manufacturer has to submit to testing.
Not all foam sold in South Africa is SABS-approved. Imported foam and foam used in lower-cost products often is not. SABS approval is a meaningful quality signal, not just a box to tick.
Shop 30 kg/m3 SABS-approved foam mattresses Full mattress buyers guide
Frequently asked questions
What foam density is best for a mattress?
30 kg/m3 SABS-approved high-density foam for the main support core. This performs for 8 to 10 years across most adult body weights. Under 25 kg/m3, expect noticeable loss of support within 3 to 5 years of regular use.
Is higher foam density always better?
For support and durability, yes. For weight, no: denser foam is heavier, which affects how easy the mattress is to move or rotate. Above 35 kg/m3, you are into specialist territory that most home sleepers do not need. 30 kg/m3 is the right balance for most residential use.
How do I find out the foam density of a mattress?
Check the product specification page for a number in kg/m3. If it is not listed, contact the manufacturer or retailer and ask directly. If they cannot give you a number, that tells you something. Reputable foam mattress manufacturers publish their density specs.
Does foam density affect how the mattress feels?
Density and feel are separate properties. A high-density foam can be made in soft, medium, or firm feel. Density affects durability and support over time. Feel (firmness) is set by the formulation and structure of the foam, independent of density. Do not confuse the two when reading mattress descriptions.
What is reconstituted foam and how does it compare on density?
Reconstituted foam (rebonded foam) is made from pressed offcuts. Its density is inconsistent because it is made from mixed-grade offscrap. Even when the measured average density looks adequate, the inconsistency means support varies across the surface and breaks down unevenly. Virgin foam at 30 kg/m3 has consistent density throughout and performs more predictably over time.