Mattress firmness scale explained
There is no standard. A 7 out of 10 from one brand can feel like a 5 from another, because nobody measures firmness the same way and no standard requires them to.
The short answer
There is no standard. A 7 out of 10 from one brand can feel like a 5 from another, because nobody measures firmness the same way and there is no British or international standard requiring them to. Use the rating to compare within a brand, never between brands.
The scale as the industry uses it
Most UK brands publish a number between 1 and 10. Broadly, this is what those numbers are meant to mean.
| Rating | Description | Who it generally suits |
|---|---|---|
| 1 to 2 | Very soft | Almost nobody. Rarely sold. |
| 3 to 4 | Soft | Lighter side sleepers, under about ten stone |
| 5 to 6 | Medium | Side sleepers and combination sleepers of average build |
| 7 to 8 | Medium firm to firm | Back sleepers, front sleepers, heavier builds |
| 9 to 10 | Extra firm | Heavier sleepers, and anyone advised to sleep firm |
Why the number is unreliable between brands
This is the part that matters, and it is demonstrable rather than a suspicion.
Otty rates its Pure Hybrid at 6.5 out of 10. Tom's Guide tested the same mattress and put it at 8.5. That is a two point gap on a ten point scale, on one mattress, between the manufacturer and an independent tester.
The reason is that there is no agreed measurement. Some brands rate by a mechanical indentation test, some by a panel of testers, some by where they want the mattress to sit in their own range. None of those produce comparable numbers, and no standard requires them to.
What this means practically
A firmness rating is useful for comparing two mattresses within one brand's range, because at least the same method produced both numbers.
Comparing a 7 from one brand against a 7 from another tells you almost nothing. If you are cross-shopping, look at construction instead: spring count, spring height, comfort layer depth and material.
Your body weight changes what a mattress feels like
The same mattress is genuinely a different firmness for different people, and this is physics rather than perception.
A mattress supports you by compressing until the resistance matches your weight. A heavier person compresses further into the mattress before that happens, so they reach the firmer layers underneath and the mattress feels softer overall. A lighter person barely compresses the comfort layers at all, so the same mattress feels firmer.
| Build | A stated 7 out of 10 will feel |
|---|---|
| Under 10 stone | Firmer than 7. Consider dropping a point or two. |
| 10 to 15 stone | About as stated. This is who the rating was set for. |
| Over 15 stone | Softer than 7. Consider going firmer than you think. |
This is why a couple with a significant weight difference find the same mattress genuinely different, and neither of them is wrong.
Sleeping position matters more than preference
What you think you like and what keeps your spine neutral are not always the same, and the second one is what you should buy.
Side sleepers need the mattress to let the shoulder and hip sink while supporting the waist, because those are the widest points of the body. Too firm and the shoulder is compressed, which is where numb arms come from. Generally 4 to 6, or higher with good zoning.
Back sleepers need the lumbar curve supported without the hips dropping. That usually means medium firm: roughly 6 to 8.
Front sleepers need the hips held up, because if they sink the lower back arches all night. Firmer is safer here: roughly 7 to 9. Front sleeping is the position most likely to cause back pain on the wrong mattress.
Combination sleepers, who move between positions, are usually best at 6 to 7, and should prioritise responsiveness so the mattress adjusts as they turn.
Soft mattresses need more springs than firm ones
This is counterintuitive and it explains a lot of bad purchases.
On a soft mattress you sink, so the mattress has to form itself around your shape. Every spring in contact with you compresses differently. That requires a high spring count, so each small region responds independently.
With too few springs, regions respond as groups. The heaviest parts of you sink together, the spine bends, and you get the hammock effect.
On a firm mattress you barely sink, so grouping never becomes an issue and a lower spring count works fine.
The practical rule
A cheap firm mattress is sound engineering. A cheap soft mattress usually is not. If you want a soft or plush feel, you need to spend on spring count, and there is no way around it.
Why a new mattress feels wrong at first
Give it at least two to four weeks before deciding, and know what to expect.
If you are coming from a sagging mattress, your body has adapted to being unsupported. A supportive mattress initially feels too firm and may produce some aching while you adjust. That usually improves within a fortnight.
New foam also softens. Comfort layers break in over the first few weeks and a mattress that feels a point too firm out of the box often settles to where it was meant to be.
This is exactly why trial length matters. A thirty day trial ends roughly when you would be able to judge properly. A hundred nights or more gives you a real answer.
Check your pillow before blaming the mattress
If your neck aches after changing mattress, the mattress may be fine.
The softer your mattress, the further your shoulder sinks into it, and the less pillow height you need to keep your neck level. Move from a firm mattress to a softer one and your existing pillow is suddenly too high. Move from a sagging mattress to a supportive one and it may now be too low.
Adjust the pillow first and give it a fortnight before drawing any conclusion about the mattress. It is a cheaper fix and it is frequently the actual problem.
How to choose firmness when you cannot try it
- Start from position, not preference. Side 4 to 6, back 6 to 8, front 7 to 9.
- Adjust for weight. Under ten stone, go a point softer. Over fifteen stone, go a point firmer.
- Ignore ratings across brands. Compare construction instead.
- Prioritise trial length over the rating. Since the number is unreliable, the ability to change your mind is worth more than the specification.
- If you are genuinely unsure, buy something you can change. An adjustable or flippable mattress removes the guess entirely, which is the only complete solution to a variable nobody measures consistently.
Questions people ask
Are mattress firmness ratings reliable?
Not between brands. Otty rates its Pure Hybrid at 6.5 out of 10 and Tom's Guide tested the same mattress at 8.5, a two point gap on a ten point scale. Some brands rate by mechanical test, some by panel, some by where they want the model to sit in their range.
What firmness should I choose for my sleeping position?
Side sleepers generally 4 to 6, back sleepers 6 to 8, front sleepers 7 to 9. Front sleeping is the position most likely to cause back pain on the wrong mattress, because if the hips sink the lower back arches all night.
Does body weight change how firm a mattress feels?
Yes, and it is physics rather than perception. A heavier person compresses further before the resistance matches their weight, reaching firmer layers, so the mattress feels softer. A lighter person barely compresses the comfort layers, so it feels firmer.
Why do soft mattresses need more springs?
Because you sink, so the mattress must form around your shape and each region needs to respond independently. With too few springs the regions respond as groups, the heaviest parts sink together and the spine bends. A cheap firm mattress is sound engineering; a cheap soft one usually is not.
How long before a new mattress feels right?
At least two to four weeks. Coming from a sagging mattress, your body has adapted to being unsupported, so a supportive one feels too firm at first. New foam also softens as comfort layers break in.