Ask three brands for a medium-firm mattress and you will be sent surfaces that measure 5, 6 and 7 on the same gauge. There is no standard, no certification, and no obligation on anyone to be consistent. Firmness is the most consequential number in a mattress purchase and it is effectively self-reported.
This is how we measure it, and how to work out what you need.
How firmness is actually measured
Ours comes off a digital compression gauge, sampled at nine points across the surface — head, centre and foot, at the left edge, centre and right edge. The reading is the force required to compress the surface by a fixed depth, converted to a ten-point scale, and then cross-checked against three testers at 135, 175 and 240 lb.
The cross-check matters more than the instrument. A gauge presses a small plate into one spot; a person distributes 175 lb over a wide area and sinks further. Where the instrument and the testers disagree, we publish the testers' consensus and say so.
The scale, in plain terms
| Rating | Description | Who it suits |
|---|---|---|
| 1–2 | Extremely soft | Nobody. Not sold by reputable brands. |
| 3–4 | Soft | Side sleepers under about 130 lb |
| 5 | Medium-soft | Side sleepers 130–180 lb |
| 5.5–6 | Medium | Side and combination sleepers 150–220 lb |
| 6.5 | Medium-firm | Back and combination sleepers 150–250 lb |
| 7 | Firm | Stomach sleepers, anyone over 250 lb |
| 8+ | Extra firm | Sleepers over 300 lb, or a specific medical instruction |
Why the same mattress feels different to different people
Because firmness is a relationship between a surface and a load, not a property of the surface.
Our 135 lb tester and our 240 lb tester almost never agree. On one medium hybrid, the lighter tester recorded excellent pressure relief and the heavier one's hips dropped 2.4 inches into the same surface. Both experiences are accurate. The mattress is a 5.5 for one of them and functionally a 4 for the other.
The practical rule that falls out of this:
- Under about 150 lb: buy one step softer than the reviews suggest
- 150–230 lb: the published rating is roughly what you will feel
- Over about 230 lb: buy one step firmer
Firmness by sleeping position
Side sleepers
Your entire body weight goes through two small contact areas: the point of the shoulder and the greater trochanter of the hip. A firm surface concentrates pressure there and you wake with a numb arm.
Aim for medium, 5 to 6, in the 150–230 lb range. The complication is that a soft enough surface to relieve the shoulder often lets the pelvis drop, which is why zoned support — firmer coils under the hips — matters more for side sleepers than for anyone else.
Back sleepers
The lumbar curve needs filling and the pelvis needs to stay level with the shoulders. Too soft and the hips sink into a hammock; too firm and the lower back is unsupported with a visible gap under it.
Medium-firm, 6 to 6.5, for almost everyone. This is the most forgiving position and the easiest to buy for.
Stomach sleepers
The riskiest position, because the pelvis is the heaviest part of the body and any sink there hyperextends the lower spine. Firm, 7, and no lower. If you are a stomach sleeper with back pain, the mattress is only part of the problem.
Combination sleepers
You need a surface that is easy to move on, which points to a responsive hybrid or latex rather than deep memory foam that you have to climb out of. Medium-firm at 6 to 6.5 is the usual answer.
Firmness and body weight, together
The two variables interact, and weight usually wins. A 240 lb side sleeper will generally be better served by a medium-firm 6.5 than the medium 5.5 the position guides recommend, because at that weight a 5.5 stops supporting the pelvis.
Break-in is real, and it is mostly you
Almost everyone moving from a soft or sagging mattress to a properly supportive one reports discomfort in the first fortnight. Your body has adapted to the old shape and it takes two to four weeks to adapt to the new one.
The mattress changes a little too — comfort foams soften by roughly 10% in the first month — but most of the adjustment is physiological. Judge a mattress at week four, not week one, and never in the first three nights.
When the firmness is genuinely wrong
Some signals do not resolve with time:
- Numb arm or aching shoulder from night one on your side. Too firm. A topper will help.
- Lower-back stiffness that gets worse across the first month. Too soft. A topper will not help; the support core is wrong.
- Waking to reposition more than three or four times. Usually too firm, occasionally too soft.
Use the trial period. Every mattress we recommend has at least 100 nights, and the the best mattresses for back pain guide covers what to do if it is the support rather than the surface that is wrong.