Mattresses are sold with firmness ratings on a numbered scale, which implies a shared measurement. No such standard exists, and the number means something different in each catalogue.
The scale is internal to each manufacturer
Firmness ratings are assigned by the maker, usually by comparison against their own range rather than against any external reference.
A model rated in the middle of one brand's range is positioned relative to that brand's softest and firmest products, which have no fixed relationship to another brand's.
The consequence is that shoppers comparing numbers across retailers are comparing labels rather than properties.
What is actually measured is indentation force
The industry does have a measurable quantity: the force required to compress a foam sample by a set proportion of its thickness.
That figure is meaningful for a material and is used in manufacturing specifications, but it describes one layer rather than an assembled mattress.
A finished mattress combines several layers, a spring unit and a cover, and the felt result depends on how they interact rather than on any single measurement.
Body weight changes the answer
Firmness is experienced as how far a person sinks before the mattress resists, which depends on how much load is applied per unit of area.
The same mattress therefore reads as firm to a lighter person and medium to a heavier one, with no disagreement between them about the object.
Manufacturers that publish weight-banded guidance are acknowledging this directly, and it is more informative than the single number on the label.
Sleeping position shifts what suits
Load distribution differs by position. Lying on the side concentrates weight at shoulder and hip, while lying on the back spreads it along the body.
A surface that supports one position comfortably can leave gaps under another, which is the practical reason position is the first question in any fitting.
Couples with different positions and different weights are the case the fixed-firmness product handles worst, which is why split constructions exist at all.
Why the number persists anyway
A single figure is easy to display, easy to filter by online and easy to compare, which suits retail even though it carries little information.
Trial periods have become the industry's actual answer, replacing specification with a return window because the specification was never going to settle the question.
The length of those trial periods reflects how long adaptation takes, which is why they are measured in weeks rather than days.