Mattress firmness is quoted on a scale of one to ten, and the scale is not standardised. One brand's "medium-firm 6" is another's 7. Worse, firmness is routinely confused with support, which is a different property entirely.
Firmness and support are not the same thing
Firmness is how the surface feels when you first lie down. It is about the comfort layers at the top.
Support is whether your spine stays in a neutral line through the night. It is about the core of the mattress and how it responds to your weight.
A soft mattress can be well supported, and a firm one can leave your hips sagging. This is why "a firm mattress is better for your back" is one of the most persistent pieces of bad advice in the category — studies comparing firm with medium-firm mattresses for back pain have generally favoured medium-firm.
The scale, roughly
- 1–2 (very soft). Rare. Significant sink.
- 3–4 (soft). Noticeable contouring. Side sleepers, lighter bodies.
- 5–6 (medium). The most-sold range. Suits most combination sleepers.
- 6–7 (medium-firm). The usual recommendation for back sleepers and for most people generally.
- 8–10 (firm). Little contouring. Front sleepers and heavier bodies.
Take these as a starting position, not a prescription, and expect the same number to differ between manufacturers.
Your sleeping position decides more than the number
Side sleepers need the shoulder and hip to sink so the spine stays straight. Too firm and the shoulder is pushed up, which is where the shoulder and neck ache comes from. Generally softer: 3–6.
Back sleepers need the lumbar curve filled without the hips dropping. Generally medium to medium-firm: 5–7.
Front sleepers need the hips held up, or the lower back over-extends all night. Generally firmer: 6–8. It is also the position most likely to cause neck problems whatever the mattress.
Combination sleepers need a surface that lets them turn. Very soft or very high-contour mattresses make turning harder, which wakes you.
Body weight changes the number
The same mattress feels different under different weights, and this is a real effect rather than a preference.
- Under about 60kg — you compress the comfort layers less, so everything feels firmer. Consider going a step softer than the general recommendation.
- 60–90kg — the range mattresses are designed around.
- Over about 90kg — you compress further into the mattress, so it feels softer and you need more support beneath. A step firmer, and pay attention to the core rather than the top layer.
What the construction tells you
Innerspring. Bouncy, cool, responsive. Pocket springs — each spring in its own fabric sleeve — isolate movement far better than open coils, which matters with two people.
Memory foam. Contours closely and isolates movement best. Slower to respond, so turning is harder, and traditional foam traps heat. Open-cell and gel-infused foams address this with partial success.
Latex. Contours less than memory foam and springs back immediately. Cooler, more durable, heavier and more expensive.
Hybrid. A pocket spring core with foam or latex comfort layers. This is where most of the market now sits, and it covers most needs adequately.
Sleeping hot
Heat is one of the most common complaints and rarely appears in the firmness figure. The pattern is straightforward: the more your body sinks into foam, the more of you is insulated.
If you sleep hot, look for springs in the core for airflow, latex rather than memory foam, and a cover that is not a thick quilted synthetic. Mattress protectors are a common hidden cause — a waterproof one is a plastic sheet, whatever the marketing says.
Trial periods matter more than the specification
It takes two to four weeks to adapt to a new mattress, and initial impressions are unreliable — a mattress that feels wrong on the first night often feels right by the third week, and occasionally the reverse.
This is why a trial period of at least 100 nights is worth more than any amount of specification reading. Check the practical terms: who collects it, what it costs, and whether a protector is required to keep the trial valid.
Nothing here is medical advice. Persistent back or neck pain is worth taking to a clinician rather than to a mattress shop.
More household notes are filed under Everyday Essentials.