Mattress firmness ratings mean very little in absolute terms. There is no industry standard that says what a 6 out of 10 feels like. One manufacturer's medium-firm is another manufacturer's firm. A mattress described as medium by a brand that tends to make soft mattresses may feel substantially firmer than a medium from a brand that runs harder.
This makes the rating itself nearly useless as a comparison tool between different brands or different product lines. It is useful only within a single range from a single manufacturer, where the relative scale at least holds internally.
What firmness actually determines
Firmness affects two things: pressure distribution and spinal alignment. A mattress that is too soft for your body weight lets your hips sink too deeply, pulling the spine out of alignment for someone who sleeps on their side or back. A mattress that is too firm does not allow the hips to sink enough, creating pressure on the hip bone and shoulder for a side sleeper.
The relevant variable for firmness is not personal preference in isolation — it is body weight in relation to mattress construction. A heavier person will compress a given foam density significantly more than a lighter person. A mattress that is medium-firm for someone weighing 70kg will often feel noticeably soft for someone weighing 95kg.
This is why firmness preference and firmness recommendation are not the same thing. Some people prefer sleeping on something that is slightly too firm for their weight because they like the feel, and the long-term cost in back discomfort may not show up for months.
Sleep position matters more than firmness preference
Side sleepers generally do better with softer to medium firmness because the shoulder and hip need to sink in somewhat to allow the spine to align horizontally.
Back sleepers generally do better with medium to medium-firm because the lumbar region needs light support without being pushed forward by a too-firm surface.
Stomach sleeping tends to cause problems with most mattresses because it places the lumbar spine in extension. If you sleep on your stomach, the firmness question is somewhat secondary to the question of whether stomach sleeping is causing problems to begin with.
Combination sleepers — which is most people — need a mattress that is adequate across positions rather than optimal for one.
Foam density and what it tells you
Memory foam and polyfoam are specified in kg per cubic metre (or in lbs per cubic foot in American product listings). Density indicates durability more than firmness — a high-density foam is not necessarily firmer than a low-density foam, but it will hold its shape longer under repeated compression.
Low-density polyfoam below about 32 kg/m3 tends to develop body impressions within two to three years of regular use. Medium density (32–48 kg/m3) lasts considerably longer. High-density memory foam (50 kg/m3 and above) retains shape well for many years under normal use.
Mattress product pages do not always list foam density. When they do not, this is worth noting as a potential sign that the construction is not something the manufacturer wants to highlight.
Trial periods
Many online mattress brands now offer trial periods of 100 to 200 nights. These are genuinely useful given the difficulty of choosing firmness from a description.
A few things worth checking before relying on a trial period: what the return process involves (some require printing labels and coordinating pickup; others arrange it entirely), whether returns are free or have a fee, and whether a refund is to original payment method or store credit.
The trial period is less useful if you need a mattress by a specific date, since the exchange timeline for a returned mattress can add weeks.
Motion transfer
For couples, motion transfer — how much movement on one side of the bed is felt on the other — is worth considering separately from firmness. Memory foam isolates motion well. Innerspring mattresses with interconnected coils transfer motion more. Pocket spring systems (individually wrapped coils) perform somewhere in between, generally closer to foam for motion isolation.
Motion transfer also depends on mattress size. A larger mattress will reduce perceived motion transfer simply by increasing the distance between sleepers.
In short
Do not compare firmness ratings across brands. Use sleep position and body weight as your starting point, not stated preference.
Check foam density if it is listed. If it is not, ask or look for independent tests that have measured it.
Trial periods are useful. Check the return terms carefully before relying on them.
If you share a bed and motion transfer is a concern, foam or pocket spring outperforms interconnected spring systems, all else equal.