Foam density, measured in kg/m³, is the single best predictor of how long a mattress will last and how well it supports your spine. For everyday adult use, look for a support core of 28 kg/m³ or higher. Under 21 kg/m³ is short-life foam, whatever the label says.
That paragraph just told you more than most mattress salespeople will tell you in an hour. The rest of this guide explains what the numbers mean, why most brands hide them, and exactly what to ask before you pay anyone a cent.
What foam density actually is
Density is the weight of one cubic metre of foam, in kilograms. A 28 kg/m³ foam means a block one metre by one metre by one metre weighs 28 kg. More material packed into the same space means the foam resists compression for longer. Less material means air, and air does not hold your spine up at 3 am.
This is why density predicts lifespan. When a mattress develops a permanent body hollow after two years, that is not bad luck. That is low-density foam doing exactly what low-density foam does.
Density is not firmness, do not let anyone blur the two
Firmness is how the bed feels on night one. Density is whether it still feels that way in year five. You can make a soft high-density foam and a firm low-density foam. The firm cheap one feels great in the showroom and collapses within 24 months. This confusion is the most profitable misunderstanding in the bed business.
The South African density table
| Support core density | What it is | Realistic lifespan | Who it suits |
|---|---|---|---|
| Under 21 kg/m³ | Entry-level foam | 18 months, 3 years | Guest bed only. Avoid for daily use. |
| 21-27 kg/m³ | Budget foam, hidden core of many "bargain" beds | 3-5 years | Kids' beds, spare rooms, tight budgets with eyes open |
| 28-34 kg/m³ | High-density foam. The sweet spot for daily adult use. | 7-10 years | Most adults. This is the band to demand. |
| 35 kg/m³ and up | Premium high-density. Maximum durability under load. | 10+ years | Heavier sleepers (100 kg+), long-term buyers |
Memory foam comfort layers run denser by nature (commonly 40 kg/m³ and up), but the comfort layer is not what fails first. The support core underneath determines the life of the mattress. Always ask about the core.
Why most brands will not tell you the number
Walk into any bed showroom and ask for the support core density in kg/m³. You will hear about "premium foam", "orthopaedic technology" and "sleep systems". Vague words are not an accident. If the number were impressive, it would be printed on the label in big letters.
We publish the density of every mattress we make. Not because we are saints, but because our numbers survive the question. When a brand hides the spec, assume the spec is the reason.
The three questions that protect you
What is the density of the support core, in kg/m³? If the answer is anything other than a number, walk.
Is that number in writing, on the spec sheet or invoice? Verbal claims evaporate at warranty time.
What sag depth does the warranty cover? A 10-year warranty that only covers a 4 cm trench is a 10-year disclaimer.
How density translates to rands
Higher density costs more to make because it is literally more material. A fair price for a high-density double bought direct from a South African manufacturer is roughly R2,400 to R5,000. When you see R12,000 on a chain store tag, the extra is mostly building, brand and commission, not foam.
What real customers say
"Engineer brain, so I compared on paper first. Mr Mattress was the only brand that publishes foam density. If you won't tell me the kg/m³, I assume it's low. Bought on that logic, sleep confirmed it."
"I'm 115 kg and every mattress I've owned developed a body hollow within a year. Five months on this foam mattress, nothing."
Rated by 10,000+ sleepers.
Frequently asked questions
What is a good foam density for a mattress?
For daily adult use, a support core of 28 kg/m³ or higher. Heavier sleepers (100 kg+) should look for 35 kg/m³ and up. Foam under 21 kg/m³ breaks down within a couple of years and should only be used for occasional guest beds.
Does higher density mean a harder mattress?
No. Density is durability, firmness is feel, and they are set independently. A high-density mattress can be soft or firm. A firm cheap mattress can still be low density, which is why it sags fast despite feeling solid in the shop.
What density foam do I need for a heavy person?
From around 100 kg of body weight, look for a support core of 35 kg/m³ or more. Lower densities compress permanently under sustained load.
How do I find out the density of a mattress?
Ask the brand directly for the support core density in kg/m³ and ask for it in writing. If you get marketing words instead of a number, treat that as your answer.
What foam density do Mr Mattress beds use?
Mr Mattress publishes the density specs of every model on its product pages. High-density SABS-approved South African foam, backed by a 10-year warranty and a 180-night trial.
How long does a high-density foam mattress last?
A support core of 28 kg/m³ or higher typically lasts 7 to 10 years with normal use. Low-density foam under 21 kg/m³ commonly fails within 18 months to 3 years.
Is memory foam density measured the same way?
Yes, also in kg/m³. Memory foam comfort layers run naturally denser, often 40 kg/m³ and up, but lifespan is decided by the support core underneath, always ask for the core density.
Written by Andries Taljaard, founder of Mr Mattress. 15+ years making foam mattresses in South Africa. Last updated: June 2026.