Mattress firmness scale explained
Firmness ratings describe how a mattress responds to a standard load, and you are not a standard load.
The thing nobody says in a shop
Firmness ratings are calibrated for roughly ten to fifteen stone.
Under that, a mattress rated 7 feels firmer than 7. Over it, the same mattress feels softer and supports less than the number implies.
There is no legal or industry standard behind the scale. Each manufacturer sets its own numbers, so a 7 from one brand is not necessarily a 7 from another.
It is still the most useful single figure published, because it is at least comparable within a range and roughly comparable between them.
The scale, as generally used
| Rating | Described as | Suits |
|---|---|---|
| 1 to 2 | Very soft | Almost nothing sold in the UK. Rare. |
| 3 to 4 | Soft | Lighter side sleepers |
| 5 to 6 | Medium | Side sleepers, lighter back sleepers |
| 6 to 7 | Medium-firm | The most widely sold. Most combination sleepers. |
| 7 to 8 | Firm | Back sleepers, heavier builds |
| 8 to 10 | Very firm | Front sleepers, much heavier builds |
Medium-firm is the most sold and the most over-recommended. It is the safest guess when nothing is known about the buyer, which is exactly why it appears everywhere and why it is frequently not the right answer for you specifically.
Working out your number
Start with your sleeping position. Side 4 to 6, back 6 to 8, front 7 to 9. Side sleepers need the shoulder and hip to sink; front sleepers need the hips held up, and those are opposite requirements.
Then adjust for weight. Under about eleven stone, go one step softer than the position suggests. Over about sixteen, one step firmer and look for 28cm depth or more.
Then account for the base. A platform top divan firms a mattress; a sprung edge divan or sprung slats soften it. The same mattress on two bases is two different beds.
If you share with someone much heavier or lighter, one surface cannot be right for both. Dual firmness or zip and link is the structural answer, and it works properly from king width upward.
Firmness is not support
The two get used interchangeably and mean different things.
Firmness is how the surface feels immediately. Support is whether your spine stays in a reasonable position all night.
A firm mattress with few springs is firm because it barely compresses, which holds you on the surface rather than following your shape. A high spring count with a soft rating contours well and can still let your hips sink.
You need firmness and construction together. Ask for the firmness number and either the spring count in your size or the foam density, because one without the other tells you very little.
Telling which way you are wrong
| What you notice | Usually means |
|---|---|
| Lower back ache on waking, eases as you move | Too soft |
| Hip or shoulder pain, side sleeping | Too firm |
| Numb arm, side sleeping | Too firm at the shoulder, or pillow too low |
| Rolling toward the middle | Too soft, or the base is sagging |
| Ache at the back of the neck | Pillow, not the mattress |
This table is more reliable than any rating, because it measures the outcome rather than the specification.
And note the direction of travel: a topper can soften a mattress but cannot firm one, because support comes from beneath you. If you are too soft, the fix is the base or a board under the mattress.
Why the trial matters more than the number
Firmness is what buyers get wrong most often and cannot be judged in a week. A mattress that feels right on night three can be wrong at month two, once your body stops compensating for whatever you slept on before.
Give any new mattress a fortnight before judging it, which is also why trials have minimum periods of around 21 to 30 nights.
An exchange beats a return. A comfort exchange lets you correct the firmness rather than start the search again, which is what most people actually need.
Related reading
Choose by position
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How this site works
Nothing here is paid for by a brand. No manufacturer has copy approval, a preview, or the ability to change a score, and where a link earns a commission that never changes a ranking.
Where a brand does not publish a specification, that is stated rather than estimated, because an absent number is itself information. The corrections log and the products I will not recommend page are both public.
Questions people ask
How does the UK mattress firmness scale work?
It runs roughly 1 to 10, with 6 to 7 medium-firm being the most widely sold. There is no legal or industry standard behind it, so each manufacturer sets its own numbers and a 7 from one brand is not necessarily a 7 from another.
What firmness should I choose?
Start from your sleeping position, side 4 to 6, back 6 to 8, front 7 to 9, then adjust for weight: one step softer under about eleven stone, one step firmer over about sixteen.
Why does the same mattress feel different to different people?
Because a firmness rating describes how a mattress responds to a standard load, and ratings are calibrated for roughly ten to fifteen stone. The same mattress is a different bed under nine stone than under eighteen.
Is firmness the same as support?
No. Firmness is how the surface feels immediately; support is whether your spine stays in a reasonable position all night. A firm mattress with few springs is firm by barely compressing, which is not the same as supporting you.