Article · Last reviewed 14 October 2026 · Nutrition and claims
What a firmness rating actually measures
Summary
A firmness number is not a measurement. There is no shared scale between manufacturers, no shared load applied to the surface and no shared instrument recording the result. Two products sold as medium-firm can differ by more than the width of the whole scale once you test them under the same body mass. What is measurable is indentation force deflection, surface temperature rise and support factor, and none of those appear on the label.
Where the number comes from
It is set commercially. A firmness rating is assigned by the manufacturer to position the product within its own range. It is internally consistent and externally meaningless, in the way that a size 10 is consistent within one label and not across a wardrobe.
There is an actual metric underneath. Indentation force deflection records the force in newtons needed to compress a foam sample by a stated percentage of its thickness, at a stated temperature. It is repeatable and comparable. It is also rarely published, because it invites comparison.
Why two mediums feel different
Support factor. The ratio between deflection at 65 per cent and at 25 per cent describes how a surface behaves as it is loaded further. A low ratio bottoms out under a heavier sleeper at the same nominal firmness.
Temperature. Foam stiffness is temperature dependent. A surface tested in a laboratory at 23 degrees Celsius is a different surface at body temperature after four hours.
What to ask instead
Three questions. What is the indentation force deflection at 25 and at 65 per cent, at what temperature was it measured, and what is the surface temperature rise after six hours at 30 watts per square metre. A supplier who has the numbers will send them.
What silence tells you. If the answer is a comfort description rather than a figure, the product has not been characterised. That is not proof it is poor. It is proof it is untested, and it should be bought on that basis.
What changed
14 October 2026: added the support factor section after two suppliers published deflection figures at both loads, and softened the claim about temperature dependence to match the range in the published data rather than a single figure.
Related Briefing issues
Issue No. 001: Room Temperature
Why the mattress is a thermal component, and what that does to a room specified on air temperature alone.
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