Firm vs Medium vs Soft: Choosing a Mattress Firmness
Firmness is how the surface feels under you. Support is whether your spine stays in line. They are different properties and a mattress can be firm and unsupportive, or soft and supportive.
The scale
There is no industry standard behind the 1–10 firmness scale — every maker sets their own. Ours works like this, and every product page on this site carries the number:
| Rating | Name | Feels like |
|---|---|---|
| 3–4 | Soft | You sink in noticeably; the surface wraps the shoulder and hip |
| 5–6 | Medium | Some give at the shoulder, the hip supported rather than cradled |
| 7–8 | Firm | You lie on the surface rather than in it |
| 9–10 | Extra firm | Almost no give; built for concentrated load |
Because the scale is not standardised, a 6 from one maker is not a 6 from another. Compare within a range, not across brands.
Sleeping position matters more than anything else
Side sleepers
On your side, the shoulder and the hip carry everything and they are the two widest points of the body. They need somewhere to go. Too firm and the spine bends upward between them; you wake with a sore shoulder or a numb arm. Most side sleepers want 4 to 6.
Back sleepers
Lying on your back, the load is spread and the gap that needs filling is the small of the back. Too soft and the hips drop, which pulls the lumbar curve flat. 5 to 7 is the usual range.
Front sleepers
On your front, any sinking at the hips arches the lower back. Front sleepers want the surface to hold them level: 7 to 8, occasionally higher.
People who change position all night
Most of us. Pick for the position you fall asleep in, and favour a mattress that responds quickly — latex or a pocket unit — so turning over does not mean climbing out of a hollow.
Weight shifts the whole scale
A firmness rating assumes an average build. Heavier sleepers compress more of the comfort layer, so a mattress feels softer to them than the number suggests; lighter sleepers never compress the comfort layer at all, so it feels firmer.
- Under about 60 kg: go one step softer than the position-based rating.
- 60 to 95 kg: the rating is calibrated for you.
- Over about 95 kg, or a combined weight above 160 kg on a shared bed: go one step firmer, and pay more attention to the support core than to the comfort layer. This is what the Premium XXL is for.
"Firm is better for your back" is not true
It is the most persistent piece of folklore in the trade, and it comes from a real observation — a collapsed, sagging mattress hurts — turned into the wrong conclusion. The problem with a sagging mattress is lost support, not lost firmness. Replacing it with something rigid can leave the lumbar spine unsupported in a different way.
What actually helps a bad back is keeping the spine in the position it holds when you stand up straight. For most people that is a medium, not a plank. If you have a diagnosed spinal condition, that is a question for your GP or physiotherapist, not for a mattress company.
When you are between two numbers
Size up on softness if you sleep on your side. Size down if you sleep on your front or your back. If two people disagree, the honest options are a reversible core — the FlexiComfort flips between about 4 and 7 — or a 6, which suits most people badly-but-equally, which is often the right compromise.
Common questions
Does a mattress soften over time?
Yes, by roughly a point over the first month as the foams settle, then very slowly. If it drops much further than that, something has failed.
How long before I know if I chose right?
Three to four weeks. The first few nights on any new mattress feel wrong because your body has adapted to the old one. That is why the trial is 100 nights rather than 14.
Can a topper change the firmness?
It can soften a firm mattress convincingly. It cannot firm up a soft one — see toppers vs pads.